Acooling fan is turned off when it is running at 850 rev/min. it turns 1500 revolutions

For this item, I would assume that we asked to determine the angular deceleration of the fan. This could be answered through the equation,
                            d = (V² - v²) / 2a 
where d is the number of revolutions made, V is the initial angular velocity, v is the final angular velocity, and a is the deceleration. Substituting, 
                             1500 = (850² - 0²) / 2(a)
                                   a = 240.83 rev/min
Thus, the fan's speed decreases by 240.83 rev/min.