Question

A and b can do a piece of work in 12 days b and c in 15 days c and a in 20 days. in how many days will a alone take to complete the work

Answer 1

a+b=12  
b+c=15
c+a=20 
(A + B)’s 1 day’s work
(B + C)’s 1 day’s work
 (C + A)’s 1 day’s work
On adding, 2(A + B + C)’s 1 day’s work
 ∴ (A + B + C)’s 1 day’s work
 ∴ A’s 1 day’s work
 ∴ A alone will complete the work in 24 days.