Question 230426
Let x = the lowest grade you can obtain on the last quiz.

Use the formula for average (add the scores up and divide by the number of scores). The last quiz, score x, will give us 5 test scores in total.

(73+75+89+91+x)/5 = 85
73+75+89+91+x=425
328+x=425
x=97

The lowest grade you can obtain is 97 if you want to have an average of at least 85.