Question:
If the thickness of a pile of 12 cardboards is 65 mm, find the thickness of a pile of 312 such cardboards.
Solution:
Let x mm be the required thickness. Then, we have:
| Thickness of cardboard (in mm) | 65 | x |
| No. of cardboards | 12 | 312 |
Clearly, when the number of cardboard is more, the thickness will also be more.
So, it is a case of direct proportion.
Now, $\frac{65}{12}=\frac{x}{312}$
$\Rightarrow x=\frac{65 \times 312}{12}$
$\Rightarrow x=1690$
Therefore, the thickness of the pile of 312 cardboards is 1690 mm.