Question

The variable Z is directly proportional to X, and inversely proportional to Y. When X is 10 and Y is 4, Z has the value 47.5.

Answer 1

Because Z is directly proportional to X and inversely proportional to Y, therefore
$Z=k( \frac{X}{Y}),\\k=constant$

When X=10 and Y=4, Z=47.5.
Therefore
k(10/4) = 47.5
   2.5k = 47.5
        k = 19.

That is,
$Z= \frac{19X}{Y}$

When X=13 and Y=10, obtain
Z = (19*13)/10 = 24.7

Answer: 24.7