$\text{a) $18^{5}$ ÷ $6^{5}$ = $3^{5}$ = 243}$
$\text{b) $21^{3}$ ÷ $7^{3}$ = $3^{3}$ = 27}$
$\text{c) $81^{5}$ ÷ $27^{3}$}$
$\text{= $(3^{4})^5$ ÷ $(3^{3})^3$}$
$\text{= $3^{20}$ ÷ $3^{9}$}$
$\text{= $3^{11}$}$
$\text{= 177147}$
$\text{d)$625^{3}$ ÷ $25^{4}$}$
$\text{= $(5^{4})^3$ ÷ $(5^{2})^4$}$
$\text{= $5^{12}$ ÷ $5^{10}$}$
$\text{= $5^{2}$ = 25}$
