\[\boxed{\text{675}\text{\ (675)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[Пусть\ a_{1},a_{2},a_{3} - углы\ данного\ треугольника.\]
\[a_{1} + a_{2} + a_{3} = 180{^\circ}.\]
\[\Longrightarrow a_{1} = x,\ \ a_{2} = x + d,\ \ a_{3} = x + 2d;\]
\[\Longrightarrow x + x + d + x + 2d = 3x + 3d = 180{^\circ} \Longrightarrow\]
\[\Longrightarrow a_{2} = x + d = 60{^\circ}.\]