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?

I did 20×2=40 because they each h ad 20 crackers.  Then I did 40×2=80 because 40 is only half the box.  There was 80 crackers in the box when it was full.

20×2=40

40×2=80