Exhibit 10.9. A tutor promises to improve GMAT scores of students by more than 50 points after three lessons. To see if this is true, the tutor takes a sample of 49 students' test scores after and before they received tutoring. The mean difference was 53 points better after tutoring, with a standard deviation of the difference equal to 12 points. Let denote the mean of the difference: score after tutoring minus score before tutoring. Refer to Exhibit 10.9. Calculate the value of the test statistic.