\[\boxed{\text{624}\text{\ (624)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[c_{n} = c_{1} \cdot q^{n - 1};\]
\[\textbf{а)}\ c_{6} = c_{1} \cdot q^{5};\]
\[\textbf{б)}\ c_{20} = c_{1} \cdot q^{19};\]
\[\textbf{в)}\ c_{125} = c_{1} \cdot q^{124};\ \]
\[\textbf{г)}\ c_{k} = c_{1} \cdot q^{k - 1};\]
\[\textbf{д)}\ c_{k + 3} = c_{1} \cdot q^{k + 2};\]
\[\textbf{е)}\ c_{2k} = c_{1} \cdot q^{2k - 1}.\]