A, Band C can do a piece of work in 11 days, 20 days and 67 days respectively, working alone. How soon can the work be done if A is assisted by Band C on alternate days? Select
A) 7.1 days
B) 8.1 days
C) 9.1 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 / 67 = 78 / 737 work done in 2 day ' s = 31 / 220 + 78 / 737 = 19 / 77 19 / 77 th work done in 2 days work done = 77 / 19 * 2 = 8.1 days answer : b"
Answer: B