Drag your cursor between the asterisks for the answer:
The answer is 12
At first sight the puzzle seems to be rather straightforward, prompting to put the number 15 at the position of the question mark. Unfortunately, in that case the pattern is broken by the lowest digit in the tree — the number 7.
In fact the rule is different: all digits from the two numbers in any two circles pointing simultaneously to a third one need to be added to produce the resulting number in that circle. For example, the top two numbers give the sum 7+2+9+9 resulting in the number 27 that fills the underlying circle. The missing number can be computed by the same rule: