The Pentagon

If you're stumped, don't worry. Here's how to fill in that last number.

From left to right, let's label the pentagons A, B, and C. The difference between the numbers in pentagons A and B can be found in Pentagon C in the same location across the board. In other words, Pentagon B - Pentagon A = Pentagon C.

For example, take the top row of numbers. A = 2, B = 5, and C = 3. Plugging in those numbers to our formula (B - A = C), we get 5 - 2 = 3, The answer then for our missing digit is 10