# Simple Algebra problem that is killing me

1. Feb 17, 2010

### albinoboy

Simple Algebra problem that is killing me!!

1. The problem statement, all variables and given/known data
From the set of equations below, involving the unknowns p, q, r, and t, find the values of t / r. (Use the correct number of significant figures.)

p = 7q
pr = qs
0.5pr^2 + 0.5qs^2 = 0.5qt^2

2. Relevant equations
~

3. The attempt at a solution
I have been working on this problem for some time now. I have tried dividing all the terms by r^2 and seting (t/r)=x just to simplify the answer. I have come up with 7.5 and -7.5 but it says these answers are incorrect.

2. Feb 17, 2010

### bfpri

Re: Simple Algebra problem that is killing me!!

Is 3.7 one of the answers? Just making sure i did it right.

3. Feb 17, 2010

### elect_eng

Re: Simple Algebra problem that is killing me!!

I got an answer very quickly doing simple substitutions into the bottom equation. A hint is to see if you can determine why s=7r, and then substitute this and the first equation into the bottom equation. Then use your trick that x=t/r . It will fall out nicely if you do this.

4. Feb 17, 2010

### albinoboy

Re: Simple Algebra problem that is killing me!!

This was a lot of help. When I got to the end I got +- (7.48331). When I inputed +- (7.5) as the answer it marked it wrong. What should the answer be if this is incorrect?

5. Feb 18, 2010

### albinoboy

Re: Simple Algebra problem that is killing me!!

Anyone?

6. Feb 18, 2010

### dacruick

Re: Simple Algebra problem that is killing me!!

fine t/r or t and r?

7. Feb 18, 2010

### dacruick

Re: Simple Algebra problem that is killing me!!

5 variables and 3 equations. one equation is second order so that probably gives 2 variables...still not enough to solve. your answer has to be in terms of a variable

8. Feb 18, 2010

### elect_eng

Re: Simple Algebra problem that is killing me!!

I got +sqrt(56) and -sqrt(56), as you did.

I guess the answer is needed to one significant figure, so maybe it should be +8 and -8.

9. Feb 18, 2010

### albinoboy

Re: Simple Algebra problem that is killing me!!

I have used up two more of my answers on:
8 and -8 it gave me this error
"Your response is within 10% of the correct value. This may be due to roundoff error, or you could have a mistake in your calculation. Carry out all intermediate results to at least four-digit accuracy to minimize roundoff error."

So I tried using four digits
7.483 and -7.483 it gave me this error
"Check the number of significant figures."

10. Feb 18, 2010

### dacruick

Re: Simple Algebra problem that is killing me!!

5 variables, not enough equations.

11. Feb 18, 2010

### elect_eng

Re: Simple Algebra problem that is killing me!!

The answers must be +7 and -7. Perhaps you need to use the first decimal to determine the roundoff direction. That is 7.483 ~ 7.4 ~ 7.

If that turns out to be incorrect, then I'm stumped.

Last edited: Feb 18, 2010
12. Feb 18, 2010

### elect_eng

Re: Simple Algebra problem that is killing me!!

He doesn't need to sove for any particular variable, but only the ratio t/r. This turns out to be possible.

13. Feb 18, 2010

### dacruick

Re: Simple Algebra problem that is killing me!!

ohh thats why i asked earlier if it was t and r or t/r. I just assumed that it was t and r because everyone was giving two answers.