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

20+20=40 I did that because that is how many they ate in all. So then 40x 2 =80 so 80 is how many were in the box. Then I subtracted 80-40 =40 I did that so could make sure 40 was half of 80 I did that because they ate 40 which is half of 80.  Because if you did 40+40 you would get 80. So 80 crackers is my answer.