8. (5 points) A fast train and a slow train are traveling towards each other. The length of the fast train is 315 meters, and the length of the slow train is 300 meters. A person sitting on the slow train sees the fast train pass by in 21 seconds. How many seconds does it take for a person sitting on the fast train to see the slow train pass by?
20
【Analysis】The time it takes for a person sitting on the slow train to see the fast train pass is 21 seconds: This is a problem of the encounter between a person and the fast train, where the person has the speed of the slow train. The encounter distance is the length of the fast train, 315 meters, and the encounter time is 21 seconds. Thus, the sum of the speeds of the person and the slow train, which is also the sum of the speeds of the fast train and the slow train, is: $315 \div 21=15$ (meters/second);

For a person sitting on the fast train to see the slow train pass, this is also an encounter problem between a person and the slow train, where the person has the speed of the fast train. The encounter distance is the length of the slow train, 300 meters. Since the two trains are moving towards each other, the speed at which a person on the train sees the other train pass is the sum of the speeds of the two trains. The time it takes for the fast train to pass the slow train can be calculated by dividing the length of the slow train by the sum of the speeds of the two trains.

【Solution】According to the problem, we have:
The sum of the speeds of the fast train and the slow train: $315 \div 21=15$ (meters/second);
The time it takes for a person sitting on the fast train to see the slow train pass is: $300 \div 15=20$ (seconds);
Answer: The time it takes for a person sitting on the fast train to see the slow train pass is 20 seconds.
Therefore, the answer is: 20.