After exercising, both Sandra and Marissa had a heart rate of 144 bpm.
Now, we will decrease this heart rate by 20 for each one as mentioned and compare to the THR:
1- Sandra:
 Heart rate after decreasing 20 bpm = 144 - 20 = 124 bpm
 Her target heart rate zone is 135 bpm—172 bpm
 Therefore, Sandra will not be in her THR zone
2- Marissa:
 Heart rate after decreasing 20 bpm = 144 - 20 = 124 bpm
 Her target heart rate zone is 143 bpm—176 bpm
 Therefore, Marissa will not be in her THR zone