What is number of birds on tree A and B

Interview Puzzle

(Q.) There are two trees A and B. Both the trees have few birds. Birds on tree A said to birds on B: -

             "If one of you come to us, we will be equal in number" 

Birds on tree B said to birds on A: -

             "If one of you come to us, we will be double to you" 

What is number of birds on tree A and B?

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Answer:- 5 Birds on A & 7 Birds on B

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