Robin can clean 72 rooms in 6 days how many can he clean in 9 days

Let's put it this way.

It's a ratio of 6:72 and we need to change it in a way to find number of rooms cleaned in 9 days.

Divide 6 & 72 both by 2

Ratio is now 3:36

Multiply 3 & 36 both by 3

Ratio is now 9:108

The number of rooms in which Robin can clean in 9 days is 108 rooms.