Time and Work - General Aptitude

Q1:

A, B and C can do a piece of work in 20, 30 and 60 days respectively. In how many days can A do the work if he is assisted by B and C on every third day?

A 12 days

B 15 days

C 16 days

D 18 days

ANS:B - 15 days

A's 2 day's work = 1 x 2 = 1 .
20 10
(A + B + C)'s 1 day's work = 1 + 1 + 1 = 6 = 1 .
20 30 60 60 10
Work done in 3 days = 1 + 1 = 1 .
10 10 5
Now, 1 work is done in 3 days.
5
 Whole work will be done in (3 x 5) = 15 days.