$\text{Đáp án:}$
$\text{Bạn tham khảo bên dưới nhé!}$
$a)$ $2$.$16$> $2^{n}$ > $4$
$2$.$2^{4}$ > $2^{n}$ > $2^{2}$
$2^{5}$> $2^{n}$ > $2^{2}$
$⇒$ $5$≥ $n$ > $2$
$⇒$ $n$ $∈$ { $3$;$4$;$5$}
$\text{Vậy n ∈ {3; 4; 5}}$
$b)$ $9$.$27$≤ $3^{n}$≤ $243$
$3^{2}$.$3^{3}$≤ $3^{n}$≤ $3^{5}$
$3^{5}$ ≤ $3^{n}$≤ $3^{5}$
$5$ ≤ $n$ ≤ $5$
$⇒$ $n$ $=$ $5$
$\text{Vậy n = 5}$