Biased, because there is only survey of one group of people (one grade, one class). The sample (the class) is too small compared to the population (assuming it’s an average school size). You will end up with less than 1 percent of the population size thus the survey is not reliable information.
1) You add 72+89 which you get 161 and you divide by two and you get 80.5 2) you do the same thing 72+89+90=251 And you divide by three and you get ≈83.66
