It would take 36 minutes. If you use unit rate, you would take 20min/5people and see how many minutes does it take to paint one wall. Which would be represented by 4min/1person. To find how many minutes it would take for 9 walls, you get 4min/1person to become 9 people for the denominator. Then, whatever you do to the denominator you do to the numerator. So, 4 times 9 is 36. Therefore, making the answer 36minutes/9people.
