1.  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

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.