Answer: 4
Step-by-step explanation: To find the second diagonal, we need to ensure that the first diagonal times 2 and the second diagonal times 2 equals 40. It needs to be multiplied by 2 because that can help us find the perimeter (example: x + x + y + y = perimeter or x times 2 plus y times 2 = perimeter). We know that one of the diagonals is 16, so to help find the perimeter, we need to multiply it by 2.
40 - 32 = 8. We now know our answer multiplied by 2, so to find the second diagonal, we can divide by 2 to get 4, so the answer is 4.