A, B and C can do a piece of work in 12, 15 and 20 days respectively. How long will they take to do it working together ?
Answers:
A can do a piece of work in 12 days
B can do the same work in 15 days
C can do the same work in 20 days
∴ A’s 1 day’s work = 1/12
B’s 1 day’s work = 1/15
C’s 1 day’s work = 1/20
∴ (A + B + C)’s together 1 day’s work = 1/12 + 1/15 + 1/20
= (5 + 4 + 3)/60
= 12/60
= 1/5
∴ They can do the work in 5 days.