y varies directly with x. When y = 20, x = 50. Find x when y = 36.
since y varies directly with x, there is some constant k such that
y = k*x
Since y=20 when x=50,
20 = k*50
k = 2/5
Now you know k, and you can figure things out for other values
36 = 2/5 * x
x = 90