18 as a decimal?

Here   $\frac{1}{8}$ in decimal means 8th part in 1.

We know that  $\frac{1}{10}$ = 0.1

Then,    $\frac{1}{8} = ?$

=>  $? = \frac{\frac{1}{8} x 0.1}{\frac{1}{10}}= \frac{0.1 x 10}{8}= \frac{0.5}{4}\frac{1}{8}= 0.125$