$$
\begin{array}{l}
(\alpha+\beta) \div 2=105^{\circ} \Rightarrow \alpha+\beta=210^{\circ} \ldots \ldots \text { (1) } \\
180^{\circ}-\beta+\gamma=\alpha(\text { adj. } \angle \text { s on st. line, ext. } \angle \text { of } \Delta) \\
\gamma=\alpha+\beta-180^{\circ} \ldots . . \text { (2) }
\end{array}
$$

Sub. (1) into (2): $\gamma=210^{\circ}-180^{\circ}=30^{\circ}$
$$
b=(210+30) \div 3=80
$$
G6.2 The average of $\alpha$ and $\beta$ is $105^{\circ}$, the average of $\alpha, \beta$ and $\gamma$ is $b^{\circ}$. Find $b$.