Explanation
Here,
Expenditure of $$4$$ members $$=Rs.\ 2800$$
So, expenditure of $$1$$ member $$=\dfrac{2800}{4}=Rs.\ 700$$
Therefore,
Expenditure of $$3$$ members $$=700\times 3=Rs.\ 2100$$
$${\textbf{Step -1: Stating the arithmetic mean of n general numbers.}}$$
$${\text{Let there be }}n{\text{ numbers }}{a_1}{\text{,}}{a_2}{\text{,}}{a_3}.....{a_n}{\text{. }}$$
$${\text{Arithmetic mean of the numbers = }}\dfrac{{{a_1} + {a_2} + {a_3}{\text{ + }}.....{\text{ + }}{a_n}{\text{.}}}}{n}$$
$${\textbf{Step -2: Finding arithmetic mean of 2 and 8.}}$$
$${\text{Here we have only two numbers - }}2,8$$
$${\text{Arithematic mean }}A = \dfrac{{2 + 8}}{2} = \dfrac{{10}}{2} = 5$$
$${\textbf{Hence, }}\ {\textbf{Arithematic mean of 8 annd 2 is 5.}}$$
Let total work be W, and work done by 1 man in 1 day be w, then
$$5\times w\times 12=W$$
Also,
$$x\times w\times 10=W$$ ( where x is number of men required to build the wall in 10 days)
Equating both equations, we get,$$x\times w\times 10=5\times w\times 12$$
$$\Rightarrow x\quad = \dfrac { 60 }{ 10 } =6\quad men$$
12 workers can complete a piece of work in 10 days
I/3 rd of original No's of workers = 12*1/3 = 4 workers
No of workers reduced then more no days are required
12*10÷ 4 = 30
Ans = 30 days
hence 20 days more
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