Converting the price per square meter to the price per linear meter.

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For a maths project, we are designing a house. One of the elements of this is the fencing, and we need to calculate the cost of fencing per linear meter. The information that I have is that a $137$mm $\times$ $23$mm plank of wood that is $5.4$ metres in length is $ \$72.63$ per square meter. How do I figure out the price of the plank of wood per linear meter?

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We need to first figure out which of the three dimensions is the one that we wish to know the price per linear meter of. Since the fence will be $5.4~\text{m}$ tall (constant), that dimension is not what we're interested in. Rather, the real question is: how wide (long) will the fence be? Can we make it go around the house? Those are the practical questions that we have in mind.

The dimension that contributes to the length of the total fence is the quantity $137~\text{mm}$. That's because the planks are usually set up so that $\text{height}>\text{width}>\text{depth}$.

Furthermore, I believe that the price of a single plank is based on the largest cross-sectional area of a single plank, that is $0.137~\text{m}\times 5.4~\text{m}$. A single plank will cost $$ (72.63~\frac{\text{\$}}{\text{m}^2}) \cdot (0.137~\text{m}\times 5.4~\text{m}) \approx \$ 53.73 $$

Sounds quite expensive. Anyway, now the price of the fence, per meter, is $$ \frac{ (72.63~\frac{\text{\$}}{\text{m}^2}) \cdot (0.137~\text{m}\times 5.4~\text{m}) }{ 0.137~\text{m} }\approx \$392.20 $$

Logically, this makes sense because we know that the fence will be 5.4 meters tall, and we know the price per square meter. So it should just be multiplied by the height in meters.