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?
How many words does Bo know? ** He knows 800*.2=<<800*.2=160>>160 words
How many words does Bo need to learn? ** So he needs to learn 800-160=<<800-160=640>>640 words
How many words does Bo need to learn a day? ** That means he needs to learn 640/40=<<640/40=16>>16 words per day
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