There are 2 trees in a garden (tree "A" and "B") and in 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 double yours.
Then the birds of tree B tell the birds of tree A that if one of you comes here, then our population will be equal to yours.
How many birds in each tree?
Source: https://www.mathsisfun.com/puzzles/birds-in-trees.html
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There are 7 birds in tree A and 5 in tree B.
A = birds in tree A
B = birds in tree B
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 double yours.
A +1 = 2(B-1)
A + 1 = 2B - 2
A + 3 = 2B
Then the birds of tree B tell the birds of tree A that if one of you comes here, then our population will be equal to yours.
A - 1 = B + 1
A - 2 = B
Substitute the 2nd equation into the first.
A + 3 = 2(A - 2)
A + 3 = 2A - 4
7 = A
Substitute this into the 1st equation.
7 + 3 = 2B
10 = 2B
B = 5
There are 7 birds in tree A and 5 birds in tree B