Subject: Find the rule

I am writing for my granddaughter, I was helping her with her math home work and she has a problem that we could not solve. Her name is Kelsay. The problem was as follows:

Find the rule: It had 13 in a circle then 2 blank circles then 31 in a circle then two more blank circles. Could you please help us.

Linda (grandma) Kelsay (student 3rd grade)

Well clearly we can have many different solutions but the problem is to try the most likely solution. Since there are two blanks between the 13 and 31 I'd say to place numbers there that are evenly spaced. That is find a number that when you add it to 13 and then add it again and again you end up at 31. So for example 4 isn't right because in two steps you only get to 13 + 4 = 17; 17 + 4 = 21; and 21 + 4 = 25 which is less than 31. 8 is too much as 13 + 8 = 21; 21 + 8 = 29; 29 + 8 = 37 which is too big.