Hunting for Averages Mollie

First, I add all the exterior numbers which = 30 in the example attached. Then, after some other tries on the website, I figured out that the exterior number total must be 3 times the interior number total. Therefore, in this example, I divided 30 by 3, which equals 10. 
My next step is to add the total of either sides, in this example it was 25 and 5. I knew the sum of both sides plus the missing number needed to add to a multiple of 4. 25+3 = 28 or 25+7 = 32
These are the only possible solutions for 25 to add a number less or equal to 10 to be divisible by 4. I tried both out. 
I then tried the other side out to check my solution and make sure both numbers created means that were accurate.