A can do a certain work in the same time in which B and C together can do it. If A and B together could do it in 10 days and C alone in 50 days, then B alone could do it in:

Answer: Option C

Explanation:

(A + B)'s 1 day's work =	1
	10

C's 1 day's work =	1
	50

(A + B + C)'s 1 day's work =		1	+	1		=	6	=	3	. .... (i)
		10		50			50		25

A's 1 day's work = (B + C)'s 1 day's work .... (ii)

From (i) and (ii), we get: 2 x (A's 1 day's work) =	3
	25

A's 1 day's work =	3	.
	50

B's 1 day's work		1	-	3		=	2	=	1	.
		10		50			50		25

So, B alone could do the work in 25 days.