Question:
68 boxes of a certain commodity require a shelf-length of 13.6 m. How many boxes of the same commodity would occupy a shelf length of 20.4 m?
Solution:
Let $x$ be the number of boxes that occupy a shelf-length of $20.4 \mathrm{~m}$.
If the length of the shelf increases, the number of boxes will also increase.
Therefore, it is a case of direct variation.
$\frac{68}{x}=\frac{13.6}{20.4}$
$\Rightarrow 68 \times 20.4=x \times 13.6$
$\Rightarrow x=\frac{68 \times 20.4}{13.6}$
$=\frac{1387.2}{13.6}$
$=102$
Thus, 102 boxes will occupy a shelf - length of $20.4 \mathrm{~m}$.