Working alone at their respective constant rates, A can complete a task in ‘a’ days and B in ‘b’ days. They take turns in doing the task with each working 3 days at a time. If A starts they finish the task in exactly 9 days. If B starts, they take a day more. How long does it take to complete the task if they both work together?	Pick
A) 8	B) 6	C) 4	D) 5	E) 9 / 2

Rationale: "work done by ab in a day = xy respectively . when a starts : no . of days when a works = 6 no . of days when b works = 3 → 6 x + 3 y = 1 when b starts : no . of days when a works = 4 no . of days when a works = 5 → 5 x + 4 y = 1 solving the above two equations for xy x = 1 / 9 y = 1 / 9 → total work done by ab in a day = 1 / 9 + 1 / 9 = 2 / 9 → no . of days to complete the work when both work together = 9 / 2 ; answer : e"
Answer: E