In a hypothesis test, we use a sample to gain insight on what is likely the outcome for the population. In your example, that is a probability of 1 out of 20 as there were 20 people in the class. A hypothesis test might test the idea that if individuals had an extra million dollars, 20% would give that money to charity. We could then test among a sample what they would do with the money and see if 20% of the sample said give it to charity.