now, if you notice the picture, the arcRSQ is 280°, since a circle has a total of 360°, then the arcRQ is 360° - 280°, or just 80°.
keeping in mind that arc's get their angle measurement from the central angle they're in, well, in this case the central angle for arcRQ is ∡ROQ, sor ∡ROQ = arcRQ.