A direct variation has the form y = kx where k is a constant, If you have one point, (x,y), you can determine the constant as follows:
k = y/x
In your case, we have the point (2.8)"
k = 8/2 = 4
So the equation for the proportion in your problem is:
y = 4x
To determine the value of y when x = 20, just plug x = 20 into the above equation. You can also set the problem up as a proportion, since the ration of y/x is a constant:
8 y
---- = ----
2 x
In your case:
8 y
---- = ------
2 20
Solve for y.
