Winter is almost here and most animals are migrating to warmer countries. There are 21 bird families living near the mountain. If 20 bird families flew away to africa and 21 bird families flew away to asia How many bird families flew away for the winter?
There are 21 bird families living near the mountain. 
20 bird families flew away to Africa. 
21 bird families flew away to Asia. 
How many bird families flew away for the winter? 

There are a total of 41 bird families that flew away for the winter.