Final answer:
After reducing their heart rates by 20 bpm from 144 bpm, Sandra's new heart rate will be outside her target zone, while Marissa's will still fall within her target zone.
Step-by-step explanation:
The question asks if Sandra or Marissa will be within their respective Target Heart Rate (THR) zones after decreasing their heart rates by 20 beats per minute (bpm) from a shared current rate of 144 bpm. Sandra's THR zone is 135-172 bpm, and Marissa's THR zone is 143-176 bpm. If both decrease their heart rate by 20 bpm, Sandra's new heart rate will be 124 bpm and Marissa's will be 124 bpm. Sandra will not be within her range as it falls below her minimum THR. However, Marissa will still be in her THR zone as 124 bpm is just below her minimum of 143 bpm.