A and B go around a circular track of length 100 m on a cycle at speeds of 18 kmph and 36 kmph. After how much time will they meet for the first time at the starting point? Pick
A) 120 sec B) 198 sec C) 178 sec D) 665 sec E) 20 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 { 100 / ( 18 * 5 / 18 ) , 100 / ( 36 * 5 / 18 ) } = lcm ( 20 , 10 ) = 20 sec . answer : e"
Answer: E