A, Band C can do a piece of work in 11 days, 20 days and 20 days respectively, working alone. How soon can the work be done if A is assisted by Band C on alternate days? Choose the most appropriate option:
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 / 20 = 31 / 220 work done in 2 day ' s = 31 / 220 + 31 / 220 = 31 / 110 31 / 110 th work done in 2 days work done = 110 / 31 * 2 = 7 days ( approx ) answer : a"
Answer: A