Question

Find y when x equals 10 if y equals eight what x equals 20

Answer 1

There can be 2 answers depending the proportionality between y and x
Assuming y is directly proportional to x
$y=kx$ where k is constant of proportionality 
$8=20k$
$k=\frac{8}{20}=\frac{2}{5}$
$y=\frac{2}{5}x$
$y=\frac{2}{5}*10$
$y=4$

Assuming y is indirectly proportional to x
$y=\frac{k}{x}$ where k is constant of proportionality 
$8=\frac{k}{20}$
$k=160$
$y=\frac{160}{x}$
$y=\frac{160}{10}$ 
$y=16$