Building one birdhouse requires 7 planks and 20 nails.  If 1 nail costs $0.05, and one plank costs $3, what is the cost, in dollars, to build 4 birdhouses?
The cost of the planks for one birdhouse is 7 * 3 = $<T>multiply(7, 3)=21
And the nails are a cost of 20 * 0.05 = $<T>multiply(20, 0.05)=1.0 for each birdhouse.
So to build one birdhouse one will need 21 + 1 = $<T>add(21, 1)=22
So the cost of building 4 birdhouses is at 4 * 22 = $<T>multiply(4, 22)=88
