if y varies directly with x, means that they can be modeled by a linear equation, lets choose a line with 0 y intercept, that is:
y = mx + b
y = mx
where m is the slope of the line, now we plug in the data we have:
y = mx
20/3 = m(30)
solving for m:
m = (20/3)(1/30)
m = 20/90
m = 2/9
so the line equation, or function modeling the y and x relationship is:
y = (2/9)x
