It takes 51 minutes for 7 people to paint 7 walls how many minutes does it take for 15 people to paint 15 walls

51 minutes7 people7 walls
x15 people15 walls

people and minutes are inversely proportional magnitudes
51 minutes * 7 people=x*15 people.

number of walls paint and minutes are directly proportional magnitudes.
51 minutes*15 walls=x * 7 walls.

therefore:
51 minutes*7 peoples*15 walls=x*15 people*7 walls
x=(51 minutes*7 peoples*15 walls) / (15 people*7 walls)=51 minutes.

answer : 51 minutes.