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 +20 and got 40 because it says both kids have half a box of crackers.Then i did 40 +40 and got 80 because it says they have half a box of crackers.

I got 80 as my answer.

20+20=40   40=40=80