Answer:
Upper left is 9, upper right is 9, lower left is 21
Explanation:
They tell us there are 9 birds that are yes fly and yes fish, so that’s the upper right.
They tell us there are 18 yes fish total. Subtract the 9 from the upper right, and that leaves 9 for the upper left.
They tell us there are 39 birds total.
39 - 18 (all fish eaters) - 4 (no fly, no fish) = 17 that can fly but don’t eat fish.
Make sense? lmk if you have questions