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? Choose the correct choice from the following choices:
A) 3 days. B) 4 days. C) 9 days. D) 8 days. E) 10 days.
Work done by X in 4 days =1/20	x 4=	1/5.   Remaining work =1 -	1/5=	4/5.   (X + Y)'s 1 day's work =1/20+1/12=8/60=2/15.   Now,	2/15	work is done by X and Y in 1 day.   So,	4/5 work will be done by X and Y in	15/2	x 4/5= 6 days.   Hence, total time taken = (6 + 4)= 
<gadget id="calculator">(6 + 4)</gadget>
<output>10</output>
10 days.   Answer: Option 10 days.
<result>E</result>
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?