Answer: Neither Sandra nor Marissa will be in her THR zone.
Explanation:
1) Actual pulse of both Sandra and Marissa : 144 bpm
2) Decrease of 20 bpm ⇒ 144 bpm - 20 bpm = 124 bpm
3) Sandra's TRH is in the range 135 - 172 bpm.
Since 124 < 135, she will be below the range.
4) Marissa's TRH range is 143 - 176 bpm.
Since, 124 < 143, she is below the range
In conlusion, neither Sandra nor Marissa will be in her THR zone.