A car when increase its normal speed by 10 km/hr,then time taken by it to cover a certain distance reduces by 1 hour.If car can achieve 120 km/hr maximum then what could be the range of time car could take with its normal speed for travelling the same distance, given that speed of car in integral value only?

Let normal speed = v, time = t, distance = d. d = v × t. At v + 10, time = t - 1: d = (v + 10)(t - 1). Equating: vt = (v + 10)(t - 1) = vt - v + 10t - 10, giving v = 10t - 10 or 10t = v + 10. With max speed 120 km/h, v ≤ 120. Since v = 10t - 10 and v must be integer, t can range from 2 to 13 hours (66 to 780 minutes). The range is (66 - 720) minutes as per option D.