Dan can do a job alone in 15 hours. Annie, working alone, can do the same job in just 10 hours. If Dan works alone for 12 hours and then stops, how many hours will it take Annie, working alone, to complete the job?	Choose the correct choice:  A) 2  B) 4  C) 6  D) 8  E) 10

Rationale: "dan can complete 1 / 15 of the job per hour . in 12 hours , dan completes 12 ( 1 / 15 ) = 4 / 5 of the job . annie can complete 1 / 10 of the job per hour . to complete the job , annie will take 1 / 5 / 1 / 10 = 2 hours . the answer is a ."
Answer: A