Jack can paint a house in 5 days, and Richard can paint the same house in 7 days. Working together, how long will it take them to finish the job?
In 7*5 = 35 days, Jack can paint 7 houses.
Here's a similar problem:
There are two small holes in a tub filled with water. If opened, one hole will empty the tub in 3 hours; the other will empty it in 6 hours. If both holes are opened at the same time, how long will it take to empty the tub?
One hole will empty the tub in 3 hours; so in 6*3 hours, it would empty 6 tubs.
From the Dr. Math archives:
Combining Rates of Work (II)
Ratio and Proportion
Ratio and Proportion: Jobs
Ratio of Rates and Work
Sergeant Marco's Maintenance Check
A Simple Formula?
A Time-Rate Problem
Word Problem with Fractions
Filling a Pool
Filling a Tank
Two Pipes and a Drain
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