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.

If they both had 20 crackers so I added 20 and 20 and got 40.  40 is half  of  the box so I added 40 and 40 and got 80.

20+20=40    40+40=80

The box had 80 crackers when it was full.