A and B can do a piece of work in 72 days, B and C can do it in 120 days, and A and C can do it in 90 days.When A, B and C work together, how much work is finished by them in 3 days?

 a) 1/40
 b) 1/30
 c) 1/20
 d) 1/10

KPSC Bevco Asst-2015

1/20.

Solution

(A+B)'s 1 day work = 1/72
(B+C)'s 1 day work = 1/120
(A+C)'s 1 day work = 1/90

On adding 2(A+B+C)'s 1 day work = 1/72 + 1/120 + 1/90 = 1/30

(A+B+C)'s 1 day work = 1/( 2 x 30)= 1/60

A,B and C together can complete work in 60 days.

Work finished by them in 3 days = 3 x 1/60 = 1/20