Suppose there are n students who take part in both the shivering competition and the hot dish competition, then there is (9-n)+6+7=20, that is, 22-n=20, so n=22-20=2.

Principle: These N people are people who take part in the cold current competition and the hot dish competition at the same time, so 9+6+7 will add those who take part in the cold current competition and the hot dish competition to 6 people and 9 people respectively, so it is necessary to subtract N once. When calculating this situation in the future, you can directly add up all the people and subtract the total number!