At first sight the puzzle seems to
be rather easy "prompting" the one to put the "15" instead of the
question mark. Unfortunately, in that case the pattern is "broken" by
the lowest digit in the tree - the "7".
In fact the rule is different. It is that all the digits from any two
circles pointing simultaneously to the third one are summed up
producing the resulting number on that circle. For example, circles
"72" and "99" are summed up in the way 7+2+9+9 and resulting in the
"27" on the circle to which the respective arrows point to. In that
way the resulting number of the circle with the question mark on it is
"12". I.e. 2+1+3+6 = 12.