We know how many pounds are needed to cover 5000 ft^2, and we need to find how many are needed to cover 27000ft^2. To find this value, divide 27000 by 5000 and multiply the result by the weight.
27000/5000 x 18.75
5.4 x 18.75
101.25
So, we need 101 1/4 pounds of fertilizer. How many bags is that?
101.25/18.75
5.4
We'll say that since the 0.4 of a bag will still need to be used, that you need 6 bags to cover 27000ft^2.