A, B and C can complete a job working individually in 10, 20 and 60 days respectively. If they  work together, in how many days will they complete the work?

A' s one days work  = 1/10

B' s one days work  = 1/20

C' s one days work  = 1/60

$A+B+C one days work=\frac{1}{10}+\frac{1}{20}+\frac{1}{60}$

$=\frac{6+3+1}{60}=\frac{10}{60}=\frac{1}{6}$

In 6 days they can complete the work