First of all, Jia's four steps are the same as Dad's three steps, that is, 54*4=2 16 and 72*3=2 16.

Then, people's footprints overlap, so every 2 16 cm, there will be six footprints in the snow (Jony J takes four steps, Dad takes three steps, the starting point is not counted, and the last step overlaps, so you will know if there is a picture).

Thirdly, 60 footprints were left in the snow, so they walked 10 times, 2 16 cm.

So the perimeter = 216 *10 = 2160cm = 21.6m ≈ 21m.