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Hello dgen,
(20) $\displaystyle m\angle AOB=70$
The angle adjacent to $\displaystyle \angle AOB$ is $\displaystyle \angle COB$.
$\displaystyle m\angle COB=110^{\circ}$ because these two angles make up a linear pair and are supplementary.
$\displaystyle \angle COB$ is a central angle and has the same measure as its intercepted arc.
Therefore, $\displaystyle m\widehat{CB}=110^{\circ}$
(24) The measure of an inscribed angle in a circle is one-half the measure of its intercepted arc.
$\displaystyle 3x+7=\frac{1}{2}(134)$
$\displaystyle 3x+7=67$
$\displaystyle 3x=60$
$\displaystyle x=20$
(26) $\displaystyle m\widehat{XY}=40^{\circ}$
$\displaystyle m\widehat{WX}=180-40=140^{\circ}$