A, B and C working together can finish a piece of work in 8 hours. A alone can do it in 20 hours and B alone can do it in 24 hours. In how many hours will C alone do the same work?
A can complete the work in $20 \mathrm{~h}$.
Work done per hour by A $=\frac{1}{20}$
B can complete the work in $24 \mathrm{~h} .$
Work done per hour by B $=\frac{1}{24}$
It takes $8 \mathrm{~h}$ to complete the work if A, B and C work together.
Work done together per hour by A,B and C $=\frac{1}{8}$
(Work done per hour by A, B and C) $=($ work done per hour by A) $+($ work done per hour by B)$+($ work done per hour by C)
OR
(Work done per hour by C) $=($ work done per hour by A, B and C) $-($ work done per hour by A)$-($ work done per hour by B)
$=\frac{1}{8}-\frac{1}{24}-\frac{1}{20}=\frac{1}{30}$
$\therefore$ C alone will take $30 \mathrm{~h}$ to complete the work.