Question 635829
In a direct variation, {{{y}}} equals {{{x}}} multiplied times a factor {{{k}}}:
{{{y=kx}}}
That way, as {{{x}}} doubles, {{{y}}} doubles; as {{{x}}} triples (or trebles, if you like that word better), {{{y}}} triples (or trebles), and so on.
The ratio is the constant {{{y/x=k}}} <---> {{{y=kx}}} .
 
It's all a question of finding your constant.
In this case,
{{{y=kx}}} and for {{{x=20}}} , {{{y=36}}} , so
{{{36=k*20}}} --> {{{k=36/20}}} --> {{{highlight(k=1.8)}}}
So {{{highlight(y=1.8x)}}} .
If your teacher would rather have fractions than decimals,
{{{k=36/20=9/5}}} and {{{y=(9/5)x}}} or {{{y=9x/5}}}