Judy spent 55 mins at gym.
Explanation:
Let the time spent by Judy be 'J'
Let the time spent by Sandy be 's'
Mary spent 3 hours at gym.
Sandy spent 50 mins less than Mary
S = 3hours - 50 mins
S = 2 hours 10 mins
Judy spent 1 hour 15 mins less than Sandy.
J = S - 1hour 15 mins
J = 2 hours 10 mins - 1 hour 15 mins
J = 55 mins
Judy spent 55 mins at gym.