The solution is 7. Each number in the tree is connected to some number in the row just below it - to some node. To solve the puzzle we have to consider every two-digit number in the tree as being just single separate two digits. That means, for example, that 16 isn't 16 but just 1 and 6. Now you can find any node (beginning from the 2nd row) by adding the individual digits from the nodes in the above row that point to it.
Thus, if we add up all the digits connected with the lines (or line) to the node just below these digits, we'll get the number for this node! For example, look at the top right numbers of the tree - 52 and 95 - which are connected with the two lines to the number just beneath them - 21. Considering 52 and 95 as four single digits - 5, 2, 9 and 5 - we'll get their sum - 5+2+9+5=21. The same is true for all remaining numbers in the tree (beginning from the 2nd row). So, finally we may see that the nodes pointing to the final answer are 13 and 3, and, therefore, instead of the question mark in the root of this number tree we have to place the number 7 what apparently is the total of 1+3+3=7.