$\begin{align}
& \overset\frown{FE}=\overset\frown{IB}=a \\
& \overset\frown{GH}=\overset\frown{BJ}=b \\
& \overset\frown{FG}={{10}^{{}^\circ }} \\
& \overset\frown{FG}+\overset\frown{GH}+\overset\frown{HJ}+\overset\frown{BJ}+\overset\frown{IB}+\overset\frown{EI}+\overset\frown{EF}+{{360}^{{}^\circ }} \\
& \Rightarrow {{10}^{{}^\circ }}+b+{{100}^{{}^\circ }}+b+a+{{90}^{{}^\circ }}+a={{360}^{{}^\circ }} \\
& \Rightarrow 2a+2b+{{200}^{{}^\circ }}={{360}^{{}^\circ }}\Rightarrow 2a+2b={{160}^{{}^\circ }}\Rightarrow a+b={{80}^{{}^\circ }} \\
& A=\frac{\overset\frown{IJ}-\overset\frown{FG}}{2}=\frac{{{80}^{{}^\circ }}-{{10}^{{}^\circ }}}{2}={{35}^{{}^\circ }} \\
\end{align}$