How many calories did you burn if you are 100 pounds and you were riding your bike for 45 mintues?

This depends on the intensity and type of exercise as well as the level of difficulty. For example 10 minutes on a treadmill might use 250 calories for one person while another would only burn 140 because they where incapable of using a more difficult setting. Also 10 minutes on a cross trainer may burn 200 Calories as it uses different muscles.

It is therefore impossible to give a answer to the above question as the number of calories burnt depends on more than just length of exercise.