Three runners Bob, Mary and James start running around a circular track of length 15m from the same point simultaneously in the same direction at speeds of 1.5 m/s, 2.5 m/s and 4.5 m/s respectively. When will they meet for the first time after they started moving? Choose the correct choice:
A) 90 seconds  B) 120 seconds  C) 30 seconds  D) 15 seconds  E) 35 seconds
If they all meet after T seconds, it means they covered the distances 1.5T, 2.5T, and 4.5T respectively.   Since they all arrive to the same spot, it means that the differences taken pairwise between the distances must be positive integer multiples of the length of the track, which is 15m. So, 1T, 2T, and 3T must all be multiples of 15. 1T multiple of 15 means T multiple of 15. The smallest T with this property is 15 and is on the list of answers.   Answer 15 seconds.
<result>D</result>
Three runners Bob, Mary and James start running around a circular track of length 15m from the same point simultaneously in the same direction at speeds of 1.5 m/s, 2.5 m/s and 4.5 m/s respectively. When will they meet for the first time after they started moving?