statistics
posted by Anonymous on .
The scatter diagram of midterm scores and final scores in a large class is football shaped. For about 80% of the students, the regression estimate of final score based on midterm score is correct to within 15 points. For about 95% of the students, the regression estimate of final score based on midterm score is correct to within how many points?

It has to be 21

why 21?

22.95

80% is between 1.2815 and 1.2815 in standard units
90% is between 1.96 and 1.96
15/r.m.s. error = 1.2815
score/r.m.s. error = 1.96
=> score = 15*1.96/1.2815
Now, when rounding 1.2815 to either 1.281 or 1.282 you'll get slightly different result