Working alone, A can complete a certain kind of job in 15 hours. A and D, working together at their respective rates, can complete one of these jobs in 10 hours. In how many hours can D, working alone, complete one of these jobs?  Pick:
A) 20 B) 24 C) 26 D) 28 E) 30

Rationale: "let total time taken by d to complete the job = d total time taken by a to complete the job = 15 work done by a in an hour 1 / a = 1 / 15 working together a and d can complete the job in 10 hours 1 / a + 1 / d = 1 / 10 = > 1 / d = 1 / 10 - 1 / 15 = 1 / 10 - 1 / 15 = 1 / 30 = > d = 30 hours answer e"
Answer: E