Dan can do a job alone in 12 hours. Annie, working alone, can do the same job in just 9 hours. If Dan works alone for 4 hours and then stops, how many hours will it take Annie, working alone, to complete the job? Choose one:
A) 2  B) 4  C) 6  D) 8  E) 10
Dan can complete 1/12 of the job per hour.   In 4 hours, Dan completes 4(1/12) = 1/3 of the job.   Annie can complete 1/9 of the job per hour.   To complete the job, Annie will take 2/3 / 1/9 = 6 hours.   The answer is 6.
<result>C</result>
Dan can do a job alone in 12 hours. Annie, working alone, can do the same job in just 9 hours. If Dan works alone for 4 hours and then stops, how many hours will it take Annie, working alone, to complete the job?