The answer is 61! Most of the answers either got it right or got very close.
The LCM of 15 and 20 is 60. This means the push-up and clapping coincide every 60 seconds. In an hour, this works out to 3600/60 = 60 times. However, Joe also did both simultaneously at the very start of the workout, so the answer is 61.
If I say that I'll perform an action every three seconds for 12 seconds, this gives one at the start, one at t = 3 seconds, one at t = 6 seconds, one at t = 9 seconds, and then the final one at t = 12 seconds, for a total of five actions. So the number of actions is t/r + 1, where t is the total time and r is the duration of the interval between each action.