There are 60 birds in three trees.After a while 4 birds fly to the second tree from the frist tree

There are 60 birds in three trees.After a while 4 birds fly to the second tree from the frist tree
and 7 birds fly to the third tree from the second tree.Then there are 20 birds in every tree.Do you know how many birds are there in every tree at first?
Key:____(the 1st tree),____(the 2nd tree),____(the 3rd tree)