A, Band C can do a piece of work in 11 days, 20 days and 55 days respectively, working alone. How soon can the work be done if A is assisted by Band C on alternate days?
Answers:  A) 7 days  B) 8 days  C) 9 days  D) 10 days  E) 11 days

Rationale: "( a + b ) ' s 1 day ' s work = 1 / 11 + 1 / 20 = 31 / 220 ( a + c ) ' s 1 day ' s work = 1 / 11 + 1 / 55 = 6 / 55 work done in 2 day ' s = 31 / 220 + 6 / 55 = 55 / 220 = 1 / 4 1 / 4 th work done in 2 days work done = 4 * 2 = 8 days answer : b"
Answer: B