Age question

  • Thread starter Natasha1
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  • #1
Natasha1
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Homework Statement


Today i am half as old as my brother was 7 years ago. I am 11 years younger than my brother. How old am i?

2. The attempt at a solution
Let I = Me and B = Brother


Let: Brother age = x
Hence my age now = x -11

So, 7 years ago:
my brother was (x - 7)
and I was (x - 11 - 7)

Lets set an equation:
1/2 x I = B
This gives us 1/2 (x - 18) = x - 7
x - 18 = 2x - 14
x = -4

Why do I get a negative age? Can anyone help solve this one please?
 

Answers and Replies

  • #2
Ray Vickson
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Homework Statement


Today i am half as old as my brother was 7 years ago. I am 11 years younger than my brother. How old am i?

2. The attempt at a solution
Let I = Me and B = Brother


Let: Brother age = x
Hence my age now = x -11

So, 7 years ago:
my brother was (x - 7)
and I was (x - 11 - 7)

Lets set an equation:
1/2 x I = B
This gives us 1/2 (x - 18) = x - 7
x - 18 = 2x - 14
x = -4

Why do I get a negative age? Can anyone help solve this one please?

It helps to keep things straight (and make your work more readable) if you have two variables: b = brother's current age, m = my current age (both in years). The conditions are that (1) m = (1/2)(b-7); and (2) m = b-11. Try solving these two equations for m and b.
 
  • #3
SammyS
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Homework Statement


Today i am half as old as my brother was 7 years ago. I am 11 years younger than my brother. How old am i?

2. The attempt at a solution
Let I = Me and B = Brother


Let: Brother age = x
Hence my age now = x -11

So, 7 years ago:
my brother was (x - 7)
and I was (x - 11 - 7)

Lets set an equation:
1/2 x I = B
This gives us 1/2 (x - 18) = x - 7
x - 18 = 2x - 14
x = -4

Why do I get a negative age? Can anyone help solve this one please?
You have that your age now is (x − 18), which is right. (See corrections below)

That is (equal to) 1/2 of what your brother's age was 7 years ago: (x − 7)

Which thing should you take 1/2 of ?

Added in Edit: Thanks Chestermiller.
Corrections:
Today you are 11 years younger than your brother (is now)
What is that? Not (x − 18) .
 
Last edited:
  • #4
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You have that your age now is (x − 18), which is right.

That is (equal to)1/2 of what your brother's age was 7 years ago: (x − 7)

Which thing should you take 1/2 of ?
Isn't his age now x-11? Isn't that half of what his brother was 7 years ago (x-7)?
 
  • #5
symbolipoint
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I will give a try but my variables choices will be different.
"Me", m
Brother, b

Today i am half as old as my brother was 7 years ago. I am 11 years younger than my brother. How old am i?

m=(1/2)(b-7)
AND
m=b-11

That system of equations is the exact translation of the description, using the variables as I've chosen.
Now, you solve that system, and you get what you get. If bad values result, then the description is at fault.
I have not tried to solve the equations yet.
 
  • #6
symbolipoint
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Again continuing with the system
m=(1/2)(b-7)
AND
m=b-11

The second equation can give b=m+11.
Substitute into the first equation and solve for m, the number being asked for in the question.
m=4.
and then b=15.

The example problem is a good typical beginners' question.
 

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