9 storks and 3 birds were sitting on the fence. 21 more birds came to join them. How many more birds than storks are sitting on the fence?
There are 9 storks and 3 birds sitting on the fence. 
21 more birds came to join them. 

That means there are now 9 storks and 24 birds sitting on the fence. 

There are 15 more birds than storks sitting on the fence.