9 men take 20 hours to paint a house. If all the men work at the same rate, how many hours would 15 men take to complete painting the house?
9 men and 15 men have a common factor of 3, so let's look at how
long 3 men would take, which would be 3 times as long. Therefore
3 men would take 60 hours to paint a house.
Therefore since 15 is 5 times as many men as 3, those 15 men would
take only one-fifth as long to paint a house as only 3 men would take.
One-fifth of 60 is 12. Therefore
15 men would take 12 hours to paint a house.
Edwin