How many bags of top soil weighing forty pounds each would it take to make one cubic yard of top soil?

A good average density for topsoil is 100 pounds per cubic foot. There are 27 cubic feet in one cubic yard, and so one cubic yard weighs around 2,700 pounds.

2,700/40 = 67.5

Therefore, it will take about 67.5 forty pound bags to make one cubic yard of topsoil.