Final answer:
The target heart rate range for a 21-year-old is 100-140 bpm.
Step-by-step explanation:
The target heart rate range for a 21-year-old is calculated based on the maximum heart rate (HR). To estimate the maximum HR, subtract the person's age (21) from 220 bpm. Then, calculate the low and high range by multiplying the estimated maximum HR by 0.5 and 0.7, respectively. Therefore, the target heart rate range for a 21-year-old would be:
- Low Range: 220 - 21 = 199 bpm x 0.5 = 99.5 bpm
- High Range: 220 - 21 = 199 bpm x 0.7 = 139.3 bpm
Therefore, the correct answer is option a) 100-140 bpm.