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?	Answers
A) 6 days	B) 10 days	C) 15 days	D) 16 days	E) 20 days
Work done by X in 4 days =(1/20)*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	(15/2x4/5)= 6 days.   Hence, total time taken = (6 + 4)= 
<gadget id="calculator">(6 + 4)</gadget>
<output>10</output>
10 days.   Option 10 days
<result>B</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?