The least common multiple of 54 and 72 is 2 16 (that is, every three steps taken by Dad coincide with every four steps taken by Didi).

After a reincarnation (that is, from their first step to coincidence), their number of steps is 6 (3+4- 1=6).

60/6= 10 cycle (at this time, Dad took 30 steps and Didi took 40 steps).

Can be calculated in two ways, the result is the same:

Dad: 10*3*72=2 160.

Didi: 10*4*54=2 160

Using two methods will improve the calculation accuracy a lot!

So the result is 2 160.