A and B go around a circular track of length 600 m on a cycle at speeds of 30 kmph and 60 kmph. After how much time will they meet for the first time at the starting point? Select:
A) 120 sec B) 176 sec C) 178 sec D) 72 sec E) 189 sec

Rationale: "time taken to meet for the first time at the starting point = lcm { length of the track / speed of a , length of the track / speed of b } = lcm { 600 / ( 30 * 5 / 18 ) , 600 / ( 60 * 5 / 18 ) } = 72 sec . answer : d"
Answer: D