Christopher can paint the interior of his house in 27 hours. If he hires Cynthia to help him they can do the same job together in 21 hours. If he lets Cynthia work alone, how long will it take her to paint the interior of his house?

Solution:

                     1/27 + 1/C = 1/21

                      1/C = 1/21   -  1/27  = 6/(21*27)

                      C  =  94.5 hours