Question

A and B can do a piece of work in 18 days. B and C in 24 days ; C and A in 20 days. In what time they can do it all working together

Answer 1

a+b =18days
b+c=24days
c+a=20days
$efficiency = \frac{1}{days}$
adding all of their efficiencies
we get
$2(a + b + c) = \frac{1}{18} + \frac{1}{24} + \frac{1}{20} \\ \: \: = \frac{20 + 15 + 18}{360} = \frac{53}{360}$
convert it to days =360/53 days
for 2(a+b+c) =360/53 days
then for a+b+c =720/53 days