Question

Kiyoshi can paint a certain fence in 3 hours by himself. If Red helps, the job takes only 2 hours. How long would it take Red to paint the fence by himself ?

Answer 1

Kiyoshi paints 1/3 of the fence each hour.

When the friends work together, they paint 1/2 of the fence each hour.

So Red must be painting (1/2 - 1/3) = 1/6 of the fence each hour.

The rate of 1/6 fence per hour means 1 fence per 6 hours for Red alone.