the ghaziabad train passes every 20 minutes the faridabad train passses every 15 minutes the gurugaon train passes every 18 minutes it is 6 p m and all the train just passed by when is the next time that will happen
LCM(20,15,18) = 2^2 * 3^2 * 5 = 180
so, the time will be 9:00 pm when it happens again.