William and Stephanie are painting the bedrooms in their house. If he had to paint the bedrooms himself, it would take William four hours to complete the job. If Stephanie alone were to paint the bedrooms, it would take her six hours.

Working together, how long will it take William and Stephanie to paint the bedrooms?

William can do 1/4 of the job every hour by himself.

Stephanie can do 1/6 of the job every hour by herself.

If they don't distract each other while working together, they can do (1/4 + 1/6) of the job every hour.

That's (3/12 + 2/12) = 5/12 job per hour working together.

The complete job will take them (12/5) hour.

That's 2-2/5 hours, or 2 hours 24 minutes.