A teacher uses an hourglass that lasts exactly five hours to measure the class time that begins at 09:00 in the morning. At a time when the teacher is on his back writing on the board, a joker student turns the clock. After a while the same student, regretted, returns it to its original position when it is 11:30.

Finally the class ends at 15:00, when all the sand in the clock has fallen.

**At what time did the student reverse the clock the first time?**

#### Solution

After turning it over twice, the hourglass continued in its initial position. If we know that the class lasted an hour longer than usual due to the two laps that were given to the hourglass, we can deduce that he counted half, 30 minutes in each change so the first time he turned around It was 30 minutes before 11:30 **at 11:00**.