To figure your target heart rate, you must take subtract your age from 220. You then need to multiply that number by .55 and .85 seperately. For example, a 25 year old would be 220-25=195. You would then take 195*.55=107.25 and then 195*.85=165.75. This example shows that this person should get his or her heart rate above 107.25 but never over 165.75. For optimal results, somewhere in between would be the best decision.