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?  Select the correct option:
A) 20
B) 25
C) 30
D) 24
E) 19
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;   2a=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 24 is answer
<result>D</result>
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?