Question

If the weight of $81$ books is $9kg$, then:

The weight of $72$ books is ____ $kg$

Answer 1

Given that,

Weight of $81$ books $=$ $9kg$

Assume that, The weight of $72$ books is $xkg$

In proportion:

$81:72::9:x$

Therefore,

$$\begin{gathered} \frac{81}{72}=\frac{9}{x} \\ x=\frac{9\times 72}{81} \\ x=8 \end{gathered}$$

Hence, The weight of $72$ books is $8$ $kg$