If $6.4$ is $80\% $ of $20\% $ of a certain number, what is the number?
প্রশ্নমতে, $x \times 20\% \times 80\% = 6.4$
$\R→ x \times \frac{20}{100} \times \frac{80}{100} = 6.4$
$\R→ x \times \frac{1}{5} \times \frac{4}{5} = 6.4$
$\R→ x \times \frac{4}{25} = 6.4$
$\R→ x = \frac{6.4 \times 25}{4} = 1.6 \times 25 = 40$।