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A mason can complete a brick wall of size $10 \text{ ft.}$ high, $15 \text{ ft }$ wide using $9 \text{ inches}$ bricks in $3 \text{ days}$. How long will be take to make $4$ walls of size $10 \text{ ft}$ high, $5 \text{ ft}$ wide using $9 \text{ inches}$ bricks?

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Mason can complete a brick wall of size $10 \text{ ft.}$ high & $15 \text{ ft.}$ wide using $9 \text{ inches} $ bricks in $3 \text{ days.}$

So, Height of the wall = $10\text{ ft.}$ i.e. $10 \times 12\text{ inch}$ = $120\text{ inch}$

Width of the wall = $15\text{ ft.}$ i.e. $15 \times 12\text{ inch}$ = $180\text{ inch}$

∴ Mason completes $120 \times 180\text{ sq. inches}$ = $21600\text{ sq. inches}$ wall in $3 \text{ days}$

∴ In 1 day Mason build $\dfrac{21600}{3}$ = $7200 \text{ sq. inches}$ brick wall using $9\text{ inches}$ bricks.

Now, Mason needs to complete $4$ brick walls of size $10 \text{ ft.}$ high & $5 \text{ ft.}$ wide

Mason needs to complete $\{4 \times (10 \times 12) \times (5 \times 12)\} = 28800 \text{ sq. inches}$ wall.

∴ $\color{green}{\text{Mason needs} \left \{ \dfrac{28800}{7200} \right \}} = \color{gold}{4\text{ days}}$ $\color{green}{\text{to complete 28800 sq.inches wall using 9 inches brick}}$

Dimensions of bricks are not changing, so it didn't affect the results & we can compute the math in **ft.** also