A, B and  C started a job which they can complete in 2 days . B can do the job in 5 days and C can do it in 4 days. After working for 1 day, both B and C left. How long would it take A to complete the rest of the job? (Unitary method)

A, B C একটি কাজ একত্রে 2 দিনে শেষ করতে পারে। B কাজটি 5 দিনে ও C কাজটি 4 দিনে শেষ করা পারে। তারা একসাথে 1 দিনে কাজ করার পর B ও C চলে গেল। বাকি কাজ A কয় দিনে করল? 

A, B & C takes 2 days to complete= 1 work

 A. B & C takes 1 days to complete = $\frac{1}{2}$ part of work

The rest = $\left(1-\frac{1}{2}\right)$=$\frac{1}{2}$  part of work

Only B takes 5 days to complete 1 work

 Only B takes 1 day to complete$\frac{1}{5}$  part of work

Only C takes 4 days to complete 1 work

Only C takes 1 day to complete$\frac{1}{4}$  part of work

Jointly (B+C) can do in I day=$\left(\frac{1}{5}+\frac{1}{4}\right)=\frac{9}{20}$ part of work

So, A can do in 1 day = $\left(\frac{1}{2}-\frac{9}{20}\right)=\frac{1}{20}$ part of work

A can do $\frac{1}{20}$ work in 1 day

A can do 1 work in 20 day 

 A can do $\frac{1}{2}$ work in $\frac{20}{2}$= 10 days