i don't know how old you are, but 20 push ups and 25 sit ups are not that much, you should already be able to do half of that. 5 weeks for the rest should be quite easy.
this may be obvious, but make sure, before the event, you can do more than required. your goal should be 30 push ups and 35 sit ups (or more) in 60 seconds, so the 20 and 25 don't tire you out.
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onward to mayhem!
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