-- 9 people paint 5 walls in 60 minutes
-- 9 people paint (5 / 5) = 1 wall in (60 / 5) = 12 minutes
-- (9 / 9) = 1 person paints 1 wall in (9 x 12) = 108 minutes
-- 1 person paints (1 x 6) = 6 walls in (108 x 6) = 648 minutes
-- (1 x 4) = 4 people paint 6 walls in (648 / 4) = 162 minutes .
