1.  X and Y can do a piece of work in 20 days and 12 days respectively. X started the work alone and then after 4 days Y joined him till the completion of the work. How long did the work last?
A. 6 days
B. 10 days
C. 15 days
D. 20 days

Answer: Option B

Explanation:

Work done by X in 4 days = 1 x 4 = 1 .
20 5

Remaining work = 1 - 1 = 4 .
5 5

(X + Y)'s 1 day's work = 1 + 1 = 8 = 2 .
20 12 60 15

Now, 2 work is done by X and Y in 1 day.
15

So, 4 work will be done by X and Y in 15 x 4
5 2 5

= 6 days.

Hence, total time taken = (6 + 4) days = 10 days.