## Elementary Statistics: Picturing the World (6th Edition)

$82.1$
$\frac{15}{100} \times (78+72+86+91+87） + \frac{25}{100} \times 80 = 82.1$ We add up the scores of the first five tests and weigh that by 15%, then weigh the score of the final test by 25% to attain our weighted mean (average).