Never. Sit ups are dangerous. Instead, substitute other exercise (such as crunches or knee-ups) to work your abs. Working your midsection or core muscle groups should not normally be done more than twice weekly.
Answer
Doing a number of pushups correctly will also improve your abs as a nice side effect!
after dinner
I'd say about 40 to 50 sit ups.
a fifteen year old should be able to do atleast 110 sit ups in two minutes.
1
60 minutes.
Sit ups do not burn stomach fat. Sit ups build and tone stomach muscles.
On the ab rocket you should do the amount of sit ups you need to until you reach you reach your limit ( your stomach starts cramping/ hurting)
5
Twice a day, do sit-ups for three minutes. Overtime, you'll get better. I can do about 65 sit-ups per minute.
sit-ups raise your heart beat more
sit ups measure your stomach strength.
5X20