For those who are having difficulty maintaining a certain number of push-ups in each set, such as doing 15 push-ups each in 5 sets, when you are doing your push-up, do not be in a rush to complete your push-up. Always take a minimum of 20 minutes to rest before you can start another set so that you will have enough strength to do the same number of push-ups you did in your first set.
I think 20mins is too much to take a rest, when you feel that tiredness while doing the push up and you don't stop that's when you get the effects more, and 30 seconds is enough for a rest. Whenever you rest for long like up to 20mins like you said, it might discourage you from continuing the task for the day, for me I do 100 every morning with some other fitness exercises, 30secs rest and I go again, like 2 to 3 times repeating same process even at night and whenever you do your exercise take a lot of water, it helps.
This challenge made some people do push-ups for the first time in their lives. If they adopt this 30-second rest before they start another set, they may want to quit this challenge because they will not have enough strength to even do up to five push-ups in the second set. If I adopt this strategy, I will not maintain my 50 push-ups in a set because my body will not get enough rest that will allow me to endure pain and still push until I reach 50 push-ups in my second set. Taking enough rest when working out is important for our bodies.