# Homework Help: Word problem.

1. Mar 24, 2010

### kee23

1. The problem statement, all variables and given/known data

Given that y is directly proportional to the cube of x and inversly proportional to the square of z, if x decreases by 23.3% and z increases by 45.3%, by what factor does y change?

2. Relevant equations
Couldn't make a equation but I tried like this:
y=K/x^3+1/z^2

3. The attempt at a solution
I couldn't get the question and where to start adding variables into the equation.
I think the equation is wrong to me though. Someone help?
(am I in the right thread for this question?)
Also my English is sort of weak so please make statement simple as possible. :)

2. Mar 24, 2010

### rock.freak667

If c is proportional to a, and proportional to b, then

c∝a and c∝b ⇒ c∝ab

3. Mar 24, 2010

### Staff: Mentor

4. Mar 24, 2010

### Rohit Mallya

5. Mar 24, 2010

### kee23

umm so I did it as y=Kx3/z2
and so I got the value : y=K12649/2052.09.
So do I replace the "y" with -23.3% and +45.3% then solve for "K"?

6. Mar 24, 2010

### kee23

umm I think so because the answer they left was in decimals so real answer is 0.214

I don't what they're saying from the question "if x decreases by 23.3% and z increases by 45.3%, by what factor does y change?"

Last edited: Mar 24, 2010
7. Mar 24, 2010

### Rohit Mallya

x decreases BY 23.3% means x=100-23.3=76.6% So you have to sub. x=76.6. But if they had given decreases to 23.3% then x=23.3
z=145.5

substituting this in the equation with k=1 we get x=21.37 in the answer which you have mentioned they have divided it by 100...

8. Mar 24, 2010

### kee23

hmmm I don't know how to get rid of "k" when I have y=K76.73/145.52

9. Mar 24, 2010

### Staff: Mentor

Look at the ratio of the new y value to the old y value. In the ratio, the K's cancel, as do the x's and z's.

BTW, 100-23.3$\neq$76.6.

10. Mar 24, 2010

### Rohit Mallya

Oops...Sorry Sir....
its 76.7...

11. Mar 24, 2010

### Rohit Mallya

yeah...Mark Sir is correct....

Eqn(1) y=kx3/y2

Eqn(2) y'=k(76.7)3/(145.5)2

Eqn(2)/Eqn(1)=> y'=21.37y

12. Mar 25, 2010

### kee23

I think I got it. Thanks for help people :D