What is the rule for the number bond diagram shown, and how do you solve for the missing number in problem 15?

To solve these problems, we need to determine the relationship between the numbers in the diagram. Based on common "number bond" or "part-part-whole" diagrams, the two numbers at the bottom usually add up to the number at the top. However, looking at problems 15 and 18, this interpretation would lead to inconsistencies or negative numbers, which is unlikely for this type of exercise.

Let's consider an alternative interpretation: one of the bottom numbers is the sum of the other bottom number and the top number. More specifically, it appears that the larger of the two bottom numbers is the sum of the smaller bottom number and the top number.

Let's verify this with , where all numbers are given:
Top number