Multiple choice general knowledge math & puzzles

There are 2 trees in a garden (tree "A" and "B") and on the both trees are some birds. The birds of tree A say to the birds of tree B that if one of you comes to our tree, then our population will be the double of yours. Then the birds of tree B tell to the birds of tree A that if one of you comes here, then our population will be equal to that of yours. How many birds are there on A & B?

  1. 9 and 6

  2. 5 and 7

  3. 7 and 5

  4. 6 and 9
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let birds on A = a, birds on B = b. If one from B goes to A: a+1 = 2(b-1) → a = 2b - 3. If one from A goes to B: b+1 = a-1 → a = b + 2. Equating: b + 2 = 2b - 3 → b = 5, then a = 7. Checking: 7+1 = 8 = 2(5-1) ✓, and 5+1 = 7-1 ✓

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