3 persons take 1 hour to complete.
Let make the house to 180u
If all paint at same speed, each paint 60u in 60 mins (1hr) but this is not.

① When J and M paint take 1h 12min = 72 mins
Therefore, we calculate in 1 hour,
J and M completed = 60÷72×180 = 150u.
So, in 1 hour S completed 180u-150u = 30u

② when M and S paint the house.
They take 1h 30 mins = 90 mins
Therefore, we calculate in 1 hour,
M and S completed = 60÷90×180 = 120u
So, in 1 hour J completed = 180u-120u = 60u

Conclusion in hour, total = 180u
J = 60u
M = 90u
S = 30u

Thus, M alone, take 2 hours (120 mins) to complete 180u.


Hope this helps