A full circle is equal to 360 degrees. The missing center piece, the arc length without an angle measurement next to it, would be the remaining angle measure that would make the full circle 360 degrees. B would be equal to half that arc angle.
Solving:
360 = B + 105 + 145
360 = B + 250
B = 110 / 2 = 55 degrees
Hope this helps!! :)