w birds were sitting on the fence. x more birds and y more storks came to join them. How many birds are sitting on the fence?
There were w birds sitting on the fence. 

x more birds came to join them. 

That makes the total number of birds sitting on the fence equal to w + x. 

y more storks came to join them. 

That makes the total number of birds sitting on the fence equal to w + x + y.