The present position of the elevator is at 10 in above the ground level.
Distance moved by the elevator below the ground level = 350 m
Therefore Total distance moved by the elevator = 350 m + 10 m = 360 m
Rate of descent = 6 m/min.
Total time taken by the elevator
=360m ÷ 6m/min = 60 minutes = 1 hour
Hence, the required time = 1 hour.