It takes 60 minutes for 4 people to paint 8 walls.

How many minutes does it take 6 people to paint 9 walls?

Each person paints 2 walls in 60 minutes (8:4=2) that means 1 wall takes 30 minutes for each person (60:2=30)

For 9 walls, each person paints 1.5 walls (9:6 = 1.5) that mean 1.5 * 30 = 45 minutes

So 6 people need 45 minutes to paint 9 walls