Munaf and Surya start simultaneously at the same point on a circular track and run along the track in the same direction. The point on the track at which they meet for the 31st  time is the same as that at which they meet for the 43rd time. The ratio of the speed of the faster boy to that of the slower one is n : 1, where n is a natural number. Which of the following is NOT a possible value of n ?