21 minutes divided by 7 walls = 3 minutes per wall for 5 people.
Assuming each worker has the same exact productivity, one person would take 15 minutes to paint a single wall (3 minutes x 5 people to get the amount of time for 1 worker per wall).
15 minutes for one worker multiplied by 5 walls = 75 minutes for one worker. Divide 75 minutes by 3 people = 25 minutes to paint all 5 walls with 3 people.
