How many minutes does it take 5 people to paint 10 walls?

It takes 35 minutes for 9 people to paint 7 walls. How many minutes does it take 5 people to paint 10 walls?

First calculate how long it will take one person to paint one wall.

It will take 

 1/7 of the time to paint one wall. So it will take 5 minutes for 9 people to paint one wall.

For only one person, it will take 9 times as long, so it will take 5⋅9=45 minutes for one person to paint one wall.

So, the number of minutes it will take for one person to paint 10 walls is 45⋅10=450.

The number of minutes it will take for 5 people to paint 10 walls is 450÷5=90.