If 3 people a,b,c can do a work simultaneously in 6 days. a and b can do work in 10 days. b and c can do work in 15 days. c and a can do the same work in 20 days.In how many days that a can complete the same work alone?
Pick one
A) 20
B) 25
C) 30
D) 24
E) 19

Rationale: a + b + c = 1 / 6 ( one day work ) a + b = 1 / 10 ; b + c = 1 / 15 ; c + a + 1 / 20 b = 1 / 15 - c ; c = 1 / 20 - a ; - - - - - - - - - ( 2 ) substitute equation 2 in a + b = 1 / 10 a = 1 / 10 - b ; a = 1 / 10 - 1 / 15 + c ; a = 1 / 10 - 1 / 15 + 1 / 20 - a ; 2 a = 1 / 10 - 1 / 15 + 1 / 20 ; a = 1 / 24 ( one day work ) that is the no of days that a can work alone is 24 days the option d is answer
Answer: D