5. Nikolai was walking to a math olympiad, covering 100 meters per minute. After 36 minutes, he stepped into School No. 15. Then he remembered that he had left the pen at home, with which he had won the school olympiad, and ran back home at a speed of 3 meters per second. How much time did he spend on the return trip?
5. 6) $100 \times 36=3600$ m - the distance from home to school;
2) $3 \times 60=180$ m - the number of meters he ran per minute on the way back;
3) $3600: 180=20$ min - the time he spent on the return trip.

Alternatively, the second step could have determined the number of seconds spent on the return trip $(3600: 3=1200$ seconds), and then convert them to minutes ( $1200: 60=20$ minutes).

In this problem, the answer can be represented in seconds, minutes, or hours.

Answer: 1200 seconds (20 min, $1 / 3$ hour).