A can do a piece of work in 4 days, B can do it in 5 days. With the assistance of C they completed the work in 2 days. Find in how many days can C alone do it.

Dear student
work done by 'A' is 4 days
work done by A in one day = 1/4
work done by B in 5  days
work done by B in one day = 1/5
work done by A, B and C is 2 days
work donr by A, B and C in 1 day = 1/2
work done by C alone in one day = work done by A, B and Cin onecday - work done by A and B in one day
                                   = $\frac{1}{2}$ - ($\frac{1}{4}$+$\frac{1}{5}$)
                                  = $\frac{1}{20}$
total work done by C is 20 days
Regards