Find the number a, if

(i) $8.4 \%$ of $a$ is 42 .

i. e., $\frac{8.4}{100} \times a=42$

$\Rightarrow a=\frac{42 \times 100}{8.4}$

$\therefore a=500$

(ii) $0.5 \%$ of $a$ is 3 .

i. e., $\frac{0.5}{100} \times a=3$

$\Rightarrow a=\frac{3 \times 100}{0.5}$

$\therefore a=600$

(iii) $\frac{1}{2} \%$ of $a$ is 50 .

i. e., $\frac{1}{2} \times \frac{1}{100} \times a=50$

$\Rightarrow a=50 \times 200$

$\therefore a=10000$

(iv) $100 \%$ of $a$ is 100 .

i. e., $\frac{100}{100} \times a=100$

$\Rightarrow 1 \times a=100$

$\therefore a=100$