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
(A+8.1 days)'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   19/77th work done in 2 days   work done= 77/19*2 = 8.1 days   ANSWER:8.1 days
<result>B</result>
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?