A mock test began at the time shown and ended the moment the hands next pointed in opposite directions. How long did the test last?

At 10:00 the minute hand is $60^{\circ }$ ahead of the hour hand (going round past 12). It must reach $180^{\circ }$ ahead, a gain of $120^{\circ }:\frac{120}{5.5}=21\frac{9}{11}$ min. (30 min ignores the hour hand's motion.)