Suppose you had a number of bedrooms all the same size.
You hose one with the blue paint and your friends hose with the other two.
In 12 hours, you hose 6 bedrooms.
In 12 hours, the friend with the green paint hoses 4 bedrooms.
In 12 hours, the third friend hoses 3 bedrooms.
In 12 hours, 13 bedrooms get hosed.
In 1 hour, how many get hosed ?
Or, how long does it take to hose 1 bedroom ?
Probably you are expected to work with fractions though (not sure).