On a physical fitness test middle school boys are awarded one point for each push-up they can do, and a point for each sit-up. National results show that the expected value is 18 pushups with a standard deviation of 4 push-ups, and an expected value of 34 sit-ups with standard deviation 11 sit-ups. The expected value of their combined (total) scores is therefore 18 + 34 = 52 points. If the number of push-ups and the number of sit-ups are independent, what is the standard deviation of their combined scores?

11.7

5.3

     15

7.2

137