\[x - ручек;\ \ y - тетрадей;\ \]
\[\ (20 - x - y) - карандашей.\]
\[5x + 3y + 2 \bullet (20 - x - y) = 50\]
\[5x + 3y + 40 - 2x - 2y = 50\]
\[3x + y = 10;\ \ \ \ (0;10) -\]
\[частное\ решение.\]
\[x = n;\ \ y = 10 - 3n;\ \ n \in Z.\]
\[Для\ x:\ \ n \geq 1;\ \ \ для\ y:\ \ \]
\[n \leq 3 \Longrightarrow n = 1;2;3.\]
\[n = 1:\]
\[x = 1;\ \ y = 10 - 3 \bullet 1 =\]
\[= 10 - 3 = 7;\ \ 20 - x - y =\]
\[= 20 - 1 - 7 = 12.\]
\[n = 2:\]
\[x = 2;\ \ y = 10 - 3 \bullet 2 =\]
\[= 10 - 6 = 4;\ \ \ 20 - x - y =\]
\[= 20 - 2 - 4 = 14.\]
\[n = 3:\]
\[x = 3;\ \ y = 10 - 3 \bullet 3 =\]
\[= 10 - 9 = 1;\ \ \ 20 - x - y =\]
\[= 20 - 3 - 1 = 16.\]
\[Ответ:ручек,\ карандашей\ и\]
\[\ тетрадей\ было\ 1,\ 7,\ 12;\ или\ \]
\[2,\ 14,\ 14;\text{\ \ \ }\ или\ 3,\ 1,\ 16.\]