20. (6 points) At 12 o'clock, the angle between the hour hand and the minute hand is 0 degrees. Thereafter, the moment when the hour hand and the minute hand form a 90-degree angle for the 6th time is $\qquad$ . (12-hour clock)
【Analysis】At 12 o'clock, the hour hand and the minute hand overlap. The minute hand moves faster than the hour hand, and the angle between the minute hand and the hour hand gradually increases from 0 degrees to 90 degrees, then to 180 degrees, and then gradually decreases to 90 degrees, and finally to 0 degrees, until the next overlap of the minute hand and the hour hand. From the overlap of the hour hand and the minute hand to the next overlap, the minute hand and the hour hand form a 90-degree angle at two moments. By estimation, between 12 o'clock and 1 o'clock, the hour hand and the minute hand form a 90-degree angle twice; between 1 o'clock and 2 o'clock, the hour hand and the minute hand form a 90-degree angle twice; a little after 2:25, the hour hand and the minute hand form a 90-degree angle for the fifth time; at 3 o'clock, the hour hand and the minute hand form a 90-degree angle for the sixth time. According to this, solve the problem.

【Solution】According to the above analysis:
Between 12 o'clock and 1 o'clock, the hour hand and the minute hand form a 90-degree angle twice; between 1 o'clock and 2 o'clock, the hour hand and the minute hand form a 90-degree angle twice; a little after 2:25, the hour hand and the minute hand form a 90-degree angle for the fifth time; at 3 o'clock, the hour hand and the minute hand form a 90-degree angle for the sixth time. Therefore, the answer is: 3 o'clock.

【Comment】The key to this problem is that the minute hand and the hour hand form a 90-degree angle twice every time they overlap again.