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

I added Davey and Charlies crackers and got forty so I would know how much was in the bottom half.  Then I wanted to know how much was in the top half I though it was the same seance it was half I added forty to forty and got eighty.So there was eighty crackers in the box when it was full.