James wants to build a 16-foot by 20-foot quilt.  He uses patches that are each 4 square feet.  The first 10 patches cost $10 each and then each patch after that cost half as much.  How much do the patches for the quilt cost?
The patch is 16*20=<T>multiply(16, 20)=320 square feet
So he needs 320/4=<T>divide(320, 4)=80.0 patches
The first 10 patches cost 10*10=$<T>multiply(10, 10)=100
He gets a discount on the next 80-10=<T>subtract(80, 10)=70 patches
They each cost 10/2=$<T>divide(10, 2)=5.0
So in total they cost 70*5=$<T>multiply(70, 5)=350
That means in total he paid 100+350=$<T>add(100, 350)=450
