# Set Up Correct Equation

Homework Statement:
Set up the correct equation leading to Peter's age now.
Relevant Equations:
2x + 5 = 3(x - 5) + 5
John is twice as old as his friend Peter. Peter is 5 years older than Alice. In 5 years, John will be three times as old as Alice. How old is Peter now? Do not solve. Just set up the correct equation.

This one involves more people.
I think a table helps out.

........................Age Now.........Age In 5 Years

John......................2x..................(2x + 5).......

Peter......................x...................(x + 5)........

Alice...................(x - 5)..............(x - 5) + 5

Equation:

##2x+5=3(x-5)+5##

Sorry but it did not display in LaTex form. I tried.

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PeroK

PeroK
Homework Helper
Gold Member
2020 Award

This is why I joined this site. If my work is not right, I need to be corrected. Be back later. Time for dinner.

PeroK
Homework Helper
Gold Member
2020 Award
This is why I joined this site. If my work is not right, I need to be corrected. Be back later. Time for dinner.
I would say that ##(x - 5) + 5 = x##, which simplifies your equation.

gmax137
There are three people
John
Peter
Alice

Pick variable names that help you keep track of what they mean. I would say
J= John's age now
P=Peter's age now
A=Alice's age now.

The first thing we are told, is "John is twice as old as his friend Peter." what does that look like in equation form?
J=2P

Can you write out equations for the other facts we are given?

"Peter is 5 years older than Alice. "
This will be an equation with P and A

In 5 years, John will be three times as old as Alice.
This will be an equation with J and A

You should end up with three equations (one for each fact). This is good, because there are three unknowns.

pbuk and harpazo
I checked the answer online. I am right.

Mark44
Mentor
Homework Statement:: Set up the correct equation leading to Peter's age now.
Relevant Equations:: 2x + 5 = 3(x - 5) + 5

Equation:
##2x+5=3(x-5)+5##

Sorry but it did not display in LaTex form. I tried.
It displayed just fine. You might have needed to refresh the screen to get the LaTeX to render.
I checked the answer online. I am right.
I don't see how. Your equation is not right. If you managed to get the correct answer starting with an incorrect equation, you probably made another error.

Using your equation (shown above), I get the ages for John, Peter, and Alice as 30, 15, and 10, respectively. In 5 years they will be 35, 20, and 15. Notice that John's age is not 3 times Alice's age.

If you correct Alice's age in 5 years to x, and multiply this by 3, you get the right answer.

It displayed just fine. You might have needed to refresh the screen to get the LaTeX to render.

I don't see how. Your equation is not right. If you managed to get the correct answer starting with an incorrect equation, you probably made another error.

Using your equation (shown above), I get the ages for John, Peter, and Alice as 30, 15, and 10, respectively. In 5 years they will be 35, 20, and 15. Notice that John's age is not 3 times Alice's age.

If you correct Alice's age in 5 years to x, and multiply this by 3, you get the right answer.
I hate to see how the author solves the problem. I like to do the math work myself, right or wrong. BRB.

It displayed just fine. You might have needed to refresh the screen to get the LaTeX to render.

I don't see how. Your equation is not right. If you managed to get the correct answer starting with an incorrect equation, you probably made another error.

Using your equation (shown above), I get the ages for John, Peter, and Alice as 30, 15, and 10, respectively. In 5 years they will be 35, 20, and 15. Notice that John's age is not 3 times Alice's age.

If you correct Alice's age in 5 years to x, and multiply this by 3, you get the right answer.

Here is what the author stated:

In 5 years, John will be three times as old as Alice.

2x + 5 = 3(x – 5 + 5)
2x + 5 = 3x

Isolate variable x
x
= 5

Answer: Peter is now 5 years old.

Mark44
Mentor
In 5 years, John will be three times as old as Alice.

2x + 5 = 3(x – 5 + 5)
Yes, and your mistake was having 3(x - 5) + 5 on the right side of the equation instead of 3(x -5 + 5). Better yet, as was suggested, it's better to simplify Alice's age in 5 years as just plain x, rather than x-5 + 5, which would make the right side of the equation above 3x.

pbuk
Gold Member
You are making this very difficult. The first step to making it easy is to use sensible letters: you are being asked to find the age of someone called Peter, why on Earth would you call that ## x ##? The next step is to write down one equation for each statement in the question, directly translating words into symbols:

John is twice as old as his friend Peter.
## j = 2p \quad (1) ## - write this down as the first line of your workings

Peter is 5 years older than Alice.
## p = a + 5 \quad (2) ## - write this down as the second line

In 5 years, John will be three times as old as Alice.
## j + 5 = 3 (a + 5) \quad (3) ## - and this is the third line.

Now we are trying to find ## p ##. Notice that equation (1) tells us that instead of j we can write 2p, and equation (2) says instead of a + 5 we can write p, so substitute equations (1) and (2) into (3) to give ## 2p + 5 = 3p ##, write this down and now we can easily solve to find ## p = 5 ##.

You are making this very difficult. The first step to making it easy is to use sensible letters: you are being asked to find the age of someone called Peter, why on Earth would you call that ## x ##? The next step is to write down one equation for each statement in the question, directly translating words into symbols:

## j = 2p \quad (1) ## - write this down as the first line of your workings

## p = a + 5 \quad (2) ## - write this down as the second line

## j + 5 = 3 (a + 5) \quad (3) ## - and this is the third line.

Now we are trying to find ## p ##. Notice that equation (1) tells us that instead of j we can write 2p, and equation (2) says instead of a + 5 we can write p, so substitute equations (1) and (2) into (3) to give ## 2p + 5 = 3p ##, write this down and now we can easily solve to find ## p = 5 ##.
I used the table method to help me set up the equation. There are several ways to answer math questions. You method may be easy for but perhaps not for others. I do agree that learning how to solve the problem via different methods is not a bad idea.

Question:

Say you came across this problem and you knew nothing about the substitution method for a system of linear equations, can this problem be solved without using algebra?

pbuk
Gold Member
I used the table method to help me set up the equation. There are several ways to answer math questions. You method may be easy for but perhaps not for others. I do agree that learning how to solve the problem via different methods is not a bad idea.
No, the reason you are taught using questions like this is to learn the basic steps of creating a mathematical model, which is to write down one equation representing each relationship or constraint specified in the problem. Writing some of the facts down in a table and hoping to extract an equation linking them all is not a viable way to solve problems in general.

In an exam there would probably be 2 marks for this question, and if you write down the equations correctly but mess up the algebra and get the answer wrong you would still get a mark. You wouldn't get anything for the table you created.

Say you came across this problem and you knew nothing about the substitution method for a system of linear equations, can this problem be solved without using algebra?
Notice that equation (1) tells us that instead of j we can write 2p, and equation (2) says instead of a + 5 we can write p.
Does this sound as scary as "the substitution method for a system of linear equations"?

No, the reason you are taught using questions like this is to learn the basic steps of creating a mathematical model, which is to write down one equation representing each relationship or constraint specified in the problem. Writing some of the facts down in a table and hoping to extract an equation linking them all is not a viable way to solve problems in general.

In an exam there would probably be 2 marks for this question, and if you write down the equations correctly but mess up the algebra and get the answer wrong you would still get a mark. You wouldn't get anything for the table you created.

Does this sound as scary as "the substitution method for a system of linear equations"?
I never had a problem using the substitution method for solving linear equations EXCEPT when it is 3 equations in 3 unknowns. In fact, I have not solve a system of linear equations in 3 unknowns in many years.

phinds
Gold Member
... I have not solve a system of linear equations in 3 unknowns in many years.
Then perhaps you should listen to @pbuk when he tells you that you are doing it the hard way. Do you seriously think he doesn't know what he is talking about? Why come here for help and then not listen to the advice you are given?

Delta2
Homework Helper
Gold Member
Your table in The OP is correct. You just did what it seems to me a minor algebra mistake (as pointed out in post #10 too) you should have stated the equation as ##2x+5=3((x-5)+5)##

It is algebra stuff that $$3(x-5)+5\neq 3((x-5)+5)$$.

harpazo and pbuk
Mark44
Mentor
No, the reason you are taught using questions like this is to learn the basic steps of creating a mathematical model, which is to write down one equation representing each relationship or constraint specified in the problem. Writing some of the facts down in a table and hoping to extract an equation linking them all is not a viable way to solve problems in general.
The table that @harpazo set up was a reasonable and viable approach, IMO. The only place he fell down was in writing the equation, mistakenly thinking that 3(x - 5) + 5 was the age of Alice in 5 years, instead of 3[(x - 5) + 5], or better yet, 3x.

pbuk
pbuk
Gold Member
Your table in The OP is correct.
The entries may be correct but they are not complete. See below.
The table that @harpazo set up was a reasonable and viable approach, IMO.
I disagree, and so do educators and examiners of material at this level.

Here is a question and an extract from the marking scheme for the November 2018 Edexcel GCSE Mathematics Foundation Paper 1 (copyright © 2018 Pearson Education Ltd, fair dealing exception claimed).

Question 13 said:
Azmol, Ryan and Kim each played a game.

Azmol’s score was four times Ryan’s score.
Kim’s score was half of Azmol’s score.

Write down the ratio of Azmol’s score to Ryan’s score to Kim’s score.

(Total for Question 13 is 2 marks)

Mark scheme said:
[Award 1 mark] for a start to express the statements ... as algebraic expressions, two of 4x, x and 2x eg 4x : x, 1x : 4x, 1x : 2x or 2x : 1x with clear and correct link to Azmol, Ryan, Kim (any letter may be used).

There are other methods that would earn partial credit (at GCSE level, age 16 all abilities, the marking schemes are very generous) but in the OP's question no marks would be awarded for the table because it does not include the relationship "John will be three times as old as Alice".

I'll say it again: answering these questions is easy if you always follow the same, correct method.
1. The unknowns in the question are always given different initial letters so use these as symbols unless told otherwise.
2. For each statement in the question in isolation, write down one expression that does nothing more than translate the words into symbols (don't try to combine information at this stage).
3. Work out a way of combining the expressions in step 2 to form an expression which only has one of the unknowns; it doesn't matter which but there is often one way to do it that works out much easier than the others (formally this is substitution but when introducing this concept I find it simpler to talk about 'replacing' j with 2p, or even 'writing 2p instead of j because we know they are the same').
Using any other method is harder, slower, prone to error and will not get partial credit.

harpazo, phinds and Delta2
jbriggs444
Homework Helper
One thing that I find important is to document your choice of variable names. I see very few students do so adequately. [And choose good names, but that's another discussion].

When learning computer programming in the 70's, we had this drilled into us. It carries over to mathematics. Before you write down an equation involving a variable x, tell us what x is.

This is crucial when you are trying to communicate with someone else (like the teacher checking your homework). It is also important for your own use. You can get buried so deep in the weeds that you can sometimes forget what you set out to accomplish in the first place.

One thing that I find important is to document your choice of variable names. I see very few students do so adequately. [And choose good names, but that's another discussion].

When learning computer programming in the 70's, we had this drilled into us. It carries over to mathematics. Before you write down an equation involving a variable x, tell us what x is.

This is crucial when you are trying to communicate with someone else (like the teacher checking your homework). It is also important for your own use. You can get buried so deep in the weeds that you can sometimes forget what you set out to accomplish in the first place.
Funny that you should say that but it all depends on the question at hand. Most of the time, I let x, y, and sometimes z be what I'm looking for.

Let x = miles driven

Let y = height of kite

Let z = amount of gas per volume

and many more....

jbriggs444
jbriggs444
Homework Helper
Mathematicians and computer programmers operate in slightly different environments. Mathematicians tend to use complex formulas with only a few different variables. They save space by using juxtaposition to indicate multiplication. That forces them to single character variable names (and subscripts, primes and greek letters). So mathematicians get used to re-using the same variable names over and over in each successive problem or theorem.

Computer programmers tend to use simpler formulas with many different variable names. Our work gets parsed by compilers that do not mind seeing explicit multiplication signs (and which can't deal well with subscripts, primes and greek letters) and can deal with large variable names without complaint. We use multi-character variable names and tend to pick new ones every time.

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PeroK, harpazo and Delta2
Staff Emeritus
The first step to making it easy is to use sensible letters: you are being asked to find the age of someone called Peter, why on Earth would you call that x?

Let a = Betty's age
Let b = Charles' age
Let c = Dorothy's age
Let d = Eugene's age
...

pbuk, phinds and jbriggs444
Let a = Betty's age
Let b = Charles' age
Let c = Dorothy's age
Let d = Eugene's age
...
Most textbooks advice to let x be the unknown number. I agree that it is best to let the variable represent what we are searching for.

Samples:

Let a = airplane

Let b = balloon

Let c = car

Let d = distance

and so forth....

I did not commit a crime here. This is math. We are having fun with numbers.

pbuk
phinds
Gold Member
We are having fun with numbers.
But it stops being fun if you manage to confuse yourself with sloppy terminology.

But it stops being fun if you manage to confuse yourself with sloppy terminology.
I understand. I joined to enjoy math not to argue. We have a choice to agree or not.