Bo is learning words for the GRE.  He got 800 flashcards.  He knew 20% of the words on them.  He has 40 days to learn the rest.  How many words does he need to learn a day?
He knows 800*.2=<T>multiply(800, 0.2)=160.0 words
So he needs to learn 800-160=<T>subtract(800, 160)=640 words
That means he needs to learn 640/40=<T>divide(640, 40)=16.0 words per day
