An angle whose vertex is on a circle and whose sides are chords of the circle is an inscribed angle.
Therefore, in the given diagram, ∠ B is an inscribed angle. The intercepted arc of this angle has a measure of 60^(∘).
According to the Inscribed Angle Theorem, the measure of an inscribed angle is half the measure of its intercepted arc.
This means that m ∠ B is half of 60^(∘).