Davey and Charlie shared half a box of crackers.  Each had 20 crackers.  How many did the box have when it was full?

First I did 20 times 2 and got 40.  I did 20 times 2 because it said they each got 20 crackers.  Then I did 40 times 2 and got 80.  I did that because they had half a box of cracker.  To get 80 I did 40 times 2=80.