Answer
16 bags
Work Step by Step
The total number of cubic feet of peat moss needed is:
$15.0ft*20.0ft*3.0in*\frac{1ft}{12in}=15.0ft*20.0ft*.25ft=75ft^3$
The total number of cubic feet in a bag of peat moss is:
$14in*\frac{1ft}{12in}*20in*\frac{1ft}{12in}*30in*\frac{1ft}{12in}=\frac{7}{6}ft*\frac{5}{3}ft*\frac{5}{2}ft=\frac{175}{36}ft^3$
Dividing $75ft^3/(\frac{175}{36}ft^3)=75*36/175\approx15.4$
However you have to have a whole number of bags (You have to round up for this problem because if you round down you won't have enough peat moss!), and therefore you need 16 bags of peat moss.
