If \( 1^2 \cdot ({^{15}C_1} )+ 2^2 \cdot ({^{15}C_2} )+ 3^2 \cdot ({^{15}C_3}) + \cdots + 15^2 \cdot ({^{15}C_{15}}) = 2^m \cdot 3^n \cdot 5^k \), where \( m,n,k \in \mathbb{N} \), then \( m + n + k \) is equal to:
\( 18 \)
\( 19 \)
\( 21 \)
\( 20 \)