\[\boxed{\mathbf{3.}\mathbf{251}}\]
\[\textbf{а)}\ 18x + 23x = 697\]
\[41x = 697\]
\[x = 697\ :41\]
\[x = 17.\ \]
\[\textbf{б)}\ 72y - 25y = 611\]
\[47y = 611\]
\[y = 611\ :47\]
\[y = 13.\]
\[\textbf{в)}\ 59z - z = 348\]
\[58z = 348\]
\[z = 348\ :58\]
\[z = 6.\]
\[\textbf{г)}\ 103t - 5t = 1960\]
\[98t = 1960\]
\[t = 1960\ :98\]
\[t = 20.\]