Question

There are two trees in a garden (A and B) and on both trees are some birds.

Answer 1

"If one of you comes to our tree, then our population will be double that of yours."

So $A+1 =2\times (B-1) A+1 = 2B - 2$

"If one of you comes here, then our population will be equal to that of yours."

$A-1 = B+1$

Rearranging these to make A the subject gives:

$A = 2B-3$
$A = B +2$

Then solve for B:

$2B - 3 = B + 2 B = 5$

Substitute B back in to an equation:

$A = B + 2 A = 5 + 2 A = 7$

So there are seven birds in A, and 5 birds in B