The PE challenge is designated to do a few at a time. The number 50 is what the end result will be. You are not expected to do 50 of each. You are not expected to 50 of one at one time. This is to build up each day to see if you can hit the goal of 50 repitions by the end of the weel. You can do variations of each exercise and switch off once you reach 50 in one of the challenges.