\[\boxed{\text{6.98.}}\]
\[\textbf{а)}\ 7,42 + 15,75 = 23,17.\ \]
\[\textbf{б)}\ 23,17 - 15,75 = 7,42.\ \]
\[\textbf{в)}\ x - 7,42 = 15,75\ \ \ \ \]
\[x = 23,17.\ \]
\[\textbf{г)}\ 7,42 + n = 23,17\ \ \ \]
\[n = 15,75.\ \]
\[\textbf{д)}\ 15,75 + z = 23,17\ \ \ \ \ \]
\[z = 7,42.\ \]
\[\textbf{е)}\ 23,17 - m = 7,42\ \ \]
\[m = 15,75.\ \ \ \]