The lowest target heart rate (THR) of a 15-year-old individual can be calculated using the following formula:
220 - age = maximum heart rate (MHR)
MHR * 50% = lowest THR
Substituting age = 15, we get:
220 - 15 = 205
205 * 50% = 102.5
Therefore, the lowest target heart rate of a 15-year-old individual is approximately 102.5 beats per minute.