math
posted by N .
If Sally can paint a house in 4 hours, and John can paint the same house in 6 hours, how long will it take for the to pain the house together?

In 24 hours, both of them can paint 6+4 houses, or 10 houses.
So the combined rate is 10houses/24hrs, or one house per 2.4 hours. 
From the information I have they are saying it should be 2 hours and 24 minutes. Can you explain?