Question

If the thickness of a pile of 12 cardboards is 65 mm, find the thickness of a pile of 312 such cardboards.

Answer 1

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.
$$\begin{gathered} \mathrm{Now},\frac{65}{12}=\frac{x}{312} \\ \Rightarrow x=\frac{65\times 312}{12} \\ \Rightarrow x=1690 \end{gathered}$$

Therefore, the thickness of the pile of 312 cardboards is 1690 mm.