Final answer:
A 15-year-old's lower part of the target heart rate zone is 123 beats per minute.
Step-by-step explanation:
To find the lower part of a 15-year-old student's target heart rate zone, we need to follow the steps provided. First, subtract the person's age (15) from 220 to get 205. This is the maximum heart rate. Next, multiply this number by 60% to find the low end of the zone, which is 123 beats per minute.