Correct Option: B

According to the question
Work done by A and B together in one day

=	1	part
	10

Work done by B and C together in one day

=	1	part
	15

Work done by C and A together in one day

=	1	part
	20

So,

A + B =	1	.........(i)
	10

B + C =	1	.........(ii)
	15

C + A =	1	.........(iii)
	20

Adding I, II, III, we get

2 (A + B + C) =	1	+	1	+	1
	10		15		20

2 (A + B + C) =	6 + 4 + 3	=	13
	60		60

A + B + C =	13	.........(iv)
	120

Putting the value of eqn. (I) in eqn. (IV)

	1	+ C =	13
	10		120

C =	13	-	1	=	13 - 12	=	1
	120		10		120		120

∴ Work done in 1 day by C is

	1	part
	120

Hence, C will finish the whole work in 120 days
Aliter : Using Rule 19,
C alone can do the work in

=	2xyz
	xy - yz + zx

=	2 × 10 × 15 × 20
	10 × 15 - 15 × 20 + 20 × 10

=	6000
	150 - 300 + 200

=	6000	= 120 days
	50