Question

. Ajay and Vijay can do a piece of work in 28 days with the help of Manoj, they can finish it in 21

days. How long will Manoj take to finish the work all alone

Answer 1

they had already finished the work so manoj had taken 0 days to do

Answer 2

→Time taken by Ajay and Vijay to do a piece of work = 28 days

→Ajay and Vijay one day's work = $\frac{1}{28}$

→Time taken by Ajay , Vijay and Manoj to do a piece of work = 21 days

→Ajay and Vijay one day's work + Manoj one day's work  = $\frac{1}{21}$

→ $\frac{1}{28}$ +  Manoj one day's work = $\frac{1}{21}$

→  Manoj one day's work = $\frac{1}{21} - \frac{1}{28}$

                                         = $\frac{28-21}{28\times21}=\frac{7}{28\times21}=\frac{7}{7\times4\times21}=\frac{1}{84}$

So, Manoj can Accomplish this work while working alone in 84 days.