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To calculate your maximum heart rate at the age of 12, subtract your age from 220.
Maximum Heart Rate = 220 - 12 = 208 beats per minute (bpm)
To find your lower range or target heart rate zone, multiply your maximum heart rate by 0.7 for the lower limit and by 0.85 for the upper limit.
Lower Range = 208 * 0.7 = 145.6 bpm
Upper Limit = 208 * 0.85 = 176.8 bpm