Working alone, A can complete a certain kind of job in 10 hours. A and D, working together at their respective rates, can complete one of these jobs in 5 hours. In how many hours can D, working alone, complete one of these jobs?
Choose the correct choice from the following answers:
A) 9 B) 10 C) 11 D) 12 E) 13

Rationale: "let total time taken by d to complete the job = d total time taken by a to complete the job = 10 work done by a in an hour 1 / a = 1 / 10 working together a and d can complete the job in 5 hours 1 / a + 1 / d = 1 / 5 = > 1 / d = 1 / 5 - 1 / a = 1 / 5 - 1 / 10 = 1 / 10 = > d = 10 hours answer b"
Answer: B