in order to achieve maximum results from aerobic exercise, one should maintain one's heart rate at a certain level.

in order to achieve maximum results from aerobic exercise, one should maintain one's heart rate at a certain level. A 35 year old woman with a resting heart rate of 85 beats per minute should keep her heart rate between 145 and 165 beats per minute while exercising. She checks her pulse for 10 seconds while exercising. What should the number of beats be?