Working alone, A can complete a certain kind of job in 3 hours. A and D, working together at their respective rates, can complete one of these jobs in 2 hours. In how many hours can D, working alone, complete one of these jobs? Select the correct option
A) 2	B) 4	C) 6	D) 8	E) 10

Rationale: let total time taken by d to complete the job = d total time taken by a to complete the job = 3 work done by a in an hour 1 / a = 1 / 3 working together a and d can complete the job in 2 hours 1 / a + 1 / d = 1 / 2 = > 1 / d = 1 / 2 - 1 / a = 1 / 2 - 1 / 3 = 1 / 6 = > d = 6 hours answer c
Answer: C