Kalon and his friend marna own a chimney sweep service company. working together, they can clean a chimney in 1 5/7 hours. if it takes kalon 4 hours to clean a 20-foot chimney by himself, how long does it take marna to clean the same size chimney by herself?

If Kalon can complete the job in 4 hours, he has completed 1 5/7 over 4 of the job if he works together with Marna. Letting x be the number of hours it will take Marna to work alone, she has completed 1 5/7 over x of the job. The amounts of job they have done should be equal to 1.
                             (1 5/7 / 4) + ( 1 5/7  / x ) = 1
The value of x is 3. So, Marna can clean the chimney in 3 hours.