To simplify \( \frac{243^3}{5} \), we first calculate \( 243^3 \).
We know that \( 243 = 3^5 \). Thus,
\[ 243^3 = (3^5)^3 = 3^{5 \cdot 3} = 3^{15}. \]
Now, we express \( \frac{243^3}{5} \) as:
\[ \frac{243^3}{5} = \frac{3^{15}}{5}. \]
This is the simplified form, so the final answer is:
\[ \frac{3^{15}}{5}. \]