Question

Abdi has two electric train sets: A and B. Each train is on its own circular track. He starts both trains at the same time. Train A returns to its starting point every 12 seconds. Train B returns to its starting point every 9 seconds. If the trains continue traveling what is the least amount of time in seconds that both trains will arrive at the starting points at the same time?

Answer 1

Answer: 36 seconds

Explanation:

A: 9, 18, 27, 36

B: 12, 24, 36

Answer 2

After 36 seconds.

Explanation:

To find the answer to this problem, we have to sum each period of time for each train, and see where they use the same time. Train A will return each 12 seconds and Train B will return each 9 seconds, this means that we have to build two sequences with this increasing numbers.

A B

12 9

24 18

36 27

48 36

As you can observe in the sequences, after 36 seconds both trains will arrive at the starting points at the same time. To reach such time equivalence, the train A has to make three laps, and train B hast to make 6 laps.

Therefore, the answer is "after 30 seconds".