I need help with the mathematical solution for this question

The freshman 15 refers to the urban legend that is the common belief that students gain an average of 15 lb during freshman year. The weights of 100 freshmen at a certain university were observed in August and then in May. The claim that the mean difference is equal to 15 lb is tested at a 0.05 significance level. The result is fail to reject the null hypothesis.

• What do the results suggest?
• What factors could have affected the results?
• How can these results be used to make changes on campus?