\[\boxed{\text{969\ (969).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[ax + by = c\]
\[(3;5):\ \ \]
\[x + 5y = c,\ \ \]
\[3 + 5 \cdot 5 = 3 + 25 = 28\ \ \]
\[тогда:\]
\[x + 5y = 28.\]