Question

Assume that A varies directly with z. If A = 30 when z = 5, find A when z = 9.

Answer 1

If A varies directly with z, and A is 30 and z is 5, then when z is 1, A is 6.
z: 5 ÷ 5 = 1
A: 30 ÷ 5 = 6
Then, when z is 1, A is 6
So, when z is 9, A is...
z: 1 × 9 = 9
A: 6 × 9 = 54

A is 54

Answer 2

We can this two ways:

1) If A = 30 when Z = 5, then A = 6Z or A = 6*z

So if Z = 9, 9 * 6 = 54. A = 54. 

2) You can set up a proportion:
$\frac{30}{5} = \frac{x}{9}$ where x = the unknown A-value.

Crossmultiply:
5x = 30*9
5x = 270

Divide by 5 on both sides to isolate the variable
x = 54

54 is your A value when Z = 9.

Either way you solve it, your answer is 54.