math
posted by maria .
Amy paints a house in 4 hours and Thomas paints the same house in 6 hours. How long will it take to paint the house together?

Here's one of Bobpursley's answers to this question.
http://www.jiskha.com/display.cgi?id=1243466404 
Amy's rate is 1/4 house per hour.
Thomas' rate is 1/6 house per hour.
The RATES are what get added when you want to know how fast they can do a job together. The combined rate is 1/4 + 1/6 = 5/12 houses per hour.
The time to do one house is 1 house divided by the rate (5/12 houses/hr), or 12/5 hours. That's 2 hours and 24 minutes.