Did you find the solution on the previous page? Here it is again in case you want another chance at it.
Here’s the solution.
The first problem is 1 + 4 = 5
The second problem is 2 + 5 (adding the previous sum: 5) = 12
The third problem is 3 + 6 (adding the previous sum: 12) = 21
The fourth problem is 8 + 11 (adding the previous sum: 21) = 40