Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.
Imagine that there are 2 trees in a garden. Let us call them Tree A and Tree B. Now imagine that there some birds on both trees.
The birds on Tree A say to the birds on Tree B: ‘If one of you comes to our tree, then our population will be the double of yours.’
Then birds on Tree B say to the birds on Tree A: ‘If one of you comes to our tree, then our population will be equal to that of yours.’
How many birds are there in each tree?
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle(UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.