To find this answer, you want to set up a proportion. The proportion should look like this:
28 40
----- = -----
x 100
Now, the fastest way to find the answer to these proportions is to find the side across (cross multiplication) that doesn't have the 'x', 28 x 100, multiply them, 2800, and look at the cross that does have the x, 40 and x. Get the other number, 40, and divide the number you got by multiplying by it. You should have the answer to x, or 70 houses total.
