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:

(a) 15 days

(b) 20 days

(c) 25 days

(d)30 days

(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 = (1/10 + 1/50) = 6/50 = 3/25 ….. (i)

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 (1/10 – 3/50) = 2/50 = 1/25.

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