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? Think carefully first, then make a decision:
He knows 800 * .2 = 160 words. So he needs to learn 800 - 160 = 640 words. That means he needs to learn 640 / 40 = 16 words per day. So, the answer is 16.