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Let us discuss time and work formulas.

If N number of people can do a piece of work in T hours, then the total effort or total work done is equal to NT.

If a person can do a work in D days then, then the work done by that person in 1 day is $\dfrac{1}{D}$.

In number ${{N}_{1}}$ of people can do ${{W}_{1}}$ amount of work in ${{D}_{1}}$ days working ${{H}_{1}}$ hours per day and ${{N}_{2}}$ number of people can do ${{W}_{2}}$ amount of work in ${{D}_{2}}$ days then

$\dfrac{{{N}_{1}}{{D}_{1}}{{H}_{1}}}{{{W}_{1}}}=\dfrac{{{N}_{2}}{{D}_{2}}{{H}_{2}}}{{{W}_{2}}}$

We use this formula and solve the problem as below.

Here ${{N}_{1}}=20$, ${{D}_{1}}=6$, ${{W}_{1}}=56$, ${{N}_{2}}=35$ and ${{D}_{2}}=3$, as the number of hours they worked per day was not given in the question, we can neglect the number of hours per day on the both sides.

We were asked to find ${{W}_{2}}$.

By applying the above problem, we get

$\begin{align}

& \dfrac{20\times 6}{56}=\dfrac{35\times 3}{{{W}_{2}}} \\

& {{W}_{2}}=\dfrac{35\times 3\times 56}{20\times 6} \\

\end{align}$

By solving this, we get

${{W}_{2}}=49$

Therefore, if 20 men can build a wall 56m long in 6 days, 35 men can build a 49m wall in 3 days.