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$5$ skilled workers can build a wall in $20$ days; $8$ semi-skilled workers can build a wall in $25$ days; $10$ unskilled workers can build a wall in $30$ days. If a team has $2$ skilled, $6$ semi-skilled and $5$ unskilled workers, how long it will take to build the wall?

1. $20$ days
2. $18$ days
3. $16$ days
4. $15$ days
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D. $15$ days

$1$ skilled person can do $\frac{1}{20 \times 5} = 1/100$ of work in $1$ day, so $2$ skilled person do $2/100$ of work in a day.

Similarly, $6$ semi-skilled and $5$ unskilled person can do $6/200$ and $5/300$ of work respectively in $1$ day.

So, together they do $\frac{2}{100}+\frac{6}{200}+{5}{300} = \frac{1}{15}$ of work together in $1$ day, which gives required number of day to complete the work $= 15.$
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