Explanation:
i) A man can do a piece of work in 20day.
So, in 1 day man can do 1/20 piece of work.
ii) In 1 day man can do 1/20 piece of work
So In 5 days man can do 1/20*5
In 5 day man can do 1/4 piece of work
iii) a man can do 1/20 piece of work in 1 day
so in 10 day a man can do 1/20*10= 1/2 work
1/2 work is left to do.
Hope this helps.
i am trying my best to give the correct answer but i am not sure this is right or not.