An electric fan is turned off, and its angular velocity decreases uniformly from 500 rev $/$ min to 200 rev $/ \min$ in 4.00 s. (a) Find the angular acceleration in rev/s $^{2}$ and the number of revolutions made by the motor in the 4.00 s interval. (b) How many more seconds are required for the fan to come to rest if the angular acceleration remains constant at the value calculated in part (a)?