Winter is almost here and most animals are migrating to warmer countries. There are w bird families living near the mountain. If x bird families flew away to africa and y bird families flew away to asia How many bird families flew away for the winter?
There are a total of w + x + y = 18 + 38 + 80 = 136 bird families. 

x bird families flew away to Africa and y bird families flew away to Asia. 

That means that w - (x + y) bird families remained near the mountain. 

w - (x + y) = 18 - (38 + 80) = 18 - 118 = -100. 

That is, there are -100 bird families remaining near the mountain. 

But that can't be right, so there must be a mistake in the problem statement.