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I do not understand why anyone is measuring what particular component of food energy is being burned. Now, When you work out, you are burning mostly carbs, namely glucose. Then once you run out of that you burn some fat too to turn into carbs. Your body runs on glucose which is a carbohydrate. Even if you burn all carbs then sit down, fat will be used to replenish the supply of glucose. So in the end you are using energy which is measured in calories. I would say 100 crunches will probably be 70 calories or so from my experience with my heart rate monitor. So concentrate on consuming less calories and burning more calories. All these people focusing on carbs have it all wrong.

It is impossible to dtermine how many carbs you burn as your body will burn some fat and some carbs with any activity. If you mean calories, even then the amount changes depending on your body weight and metabolism etc. If you want to burn the most calories, it has been shown that weight training combined with cardio and a sensible eating routine is the fastest way.

That will differ depending on your body size, weight, level of activity, metabolism. There just isn't a good answer to this. And if you are thinking of a low carb diet, think carefully about it. It did NOT work for me. What works is burning more calories than you take in. Your body needs a balanced diet to function properly.