Mona and Sona go around a circular track of length 400 m on a bike 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  B) 80  C) 60  D) 136  E) 150

Rationale: time taken to meet for the first time at the starting point = lcm { length of the track / speed of mona , length of the track / speed of sona } = lcm { 400 / ( 18 * 5 / 18 ) , 400 / ( 36 * 5 / 18 ) } = lcm ( 80 , 40 ) = 80 sec . answer : b
Answer: B