1 min 20 sec =80 sec?

One minute and thirty seconds equals ninety seconds?

80,90=720?

720 seconds = 12 minutes

Or:

1 min 12 seconds is 72 seconds, 1 min 20 seconds is 80 seconds, 1 min 30 seconds is 90 seconds.

Least common multiple of 90, 80 and 72: (90, 80, 72)=720.

720 seconds = 12 minutes;

A: At least after 12 minutes, the three met at the same time at the starting point.

So the answer is: 12.

Extended data:

The property between the greatest common factor and the least common multiple: the product of two natural numbers is equal to the product of the greatest common factor and the least common multiple of these two natural numbers. The calculation of the least common multiple is to find all common prime factors and unique prime factors of three numbers, and finally divide them into pairwise coprime.

Baidu encyclopedia-least common multiple