One maid can clean the house three times faster than another. Working together they can clean the entire house in three hours. How long would it take the faster maid cleaning alone?

Let x = time (hours) it takes faster maid

then

3x = time (hours) it takes slower maid

.

3(1/x + 1/(3x)) = 1

3(3 + 1) = 3x

3(4) = 3x

4 hours = x