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 got 80 crackers because they said they shared a half of the package and they each had 20 crackers.  I added 20 and 20 because they each had 20, and there was 2 kids.  Then I did 40 and 20 and got 60 because that is a part of the package.   Then I added 60 and 20 and it was 80 because that is the last of the package.