1.  A and B can do a piece of work in 30 days, while B and C can do the same work in 24 days and C and A in 20 days. They all work together for 10 days when B and C leave. How many days more will A take to finish the work?
A. 18 days
B. 24 days
C. 30 days
D. 36 days

Answer: Option A

Explanation:

2(A + B + C)'s 1 day's work

= ( 1 + 1 + 1 ) = 15 = 1 .
30 24 20 120 8

Therefore, (A + B + C)'s 1 day's work

= 1 = 1 .
2 x 8 16

Work done by A, B, C in 10 days = 10 = 5 .
16 8

Remaining work = ( 1 - 5 ) = 3 .
8 8

A's 1 day's work = ( 1 - 1 ) = 1 .
16 24 48

Now, 1 work is done by A in 1 day.
48

So, 3 work will be done by A in ( 48 x 3 )
8 8

= 18 days.