Working alone, A can complete a certain kind of job in 6 hours. A and D, working together at their respective rates, can complete one of these jobs in 4 hours. In how many hours can D, working alone, complete one of these jobs?
Choose the correct answer:	A) 8	B) 10	C) 12	D) 14	E) 16

Rationale: "let total time taken by d to complete the job = d total time taken by a to complete the job = 6 work done by a in an hour 1 / a = 1 / 6 working together a and d can complete the job in 4 hours 1 / a + 1 / d = 1 / 4 = > 1 / d = 1 / 4 - 1 / a = 1 / 4 - 1 / 6 = 1 / 12 = > d = 12 hours answer c"
Answer: C