Q: How many calories do you burn doing 15 jumping jacks

A: Very very few. I don't know exactly, but 2 or 3 calories at most. Don't think of jumping jacks or any other one exercise in terms of burning calories while doing the exercise (although they certainly will burn some calories). Instead, take the holistic view and understand that by constructing a heavy whole-body resistance training program along with a good diet for yourself you will greatly boost the total number of calories burned not just during the exercise, but also AFTER it as your body repairs the muscle damage that the exercise inflicts.