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=
<gadget id="calculator">800*.2</gadget>
<output>160</output>
160 words.
So he needs to learn 800-160=
<gadget id="calculator">800-160</gadget>
<output>640</output>
640 words.
That means he needs to learn 640/40=
<gadget id="calculator">640/40</gadget>
<output>16</output>
16 words per day.

<result>16</result>