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For a 13-year-old, the estimated maximum heart rate is 207 beats per minute.
To calculate the lower range of the target heart rate zone, you would first find 70-85% of the maximum heart rate:
70% of 207 = 0.7 x 207 = 144.9
85% of 207 = 0.85 x 207 = 175.95
So, the lower range of the target heart rate zone would be approximately 145 beats per minute.