How many calories does a 100 lb guy burn by doing one push ups
That's a vague question. A person will burn more calories during pushup #19 of 20, than #1 of 20.
Have you warmed up first? Have you stretched? What is your current heart-rate? Which STYLE of pushup are you doing? Are you using pushup bars? Are your knees (girly-style) or on your toes? Are your feet apart or together?