The average of 6 quiz scores, where [tex]x[/tex] is the score of the last quiz, is
[tex]\dfrac{83+81+89+92+80+x}6=\dfrac{425+x}6[/tex]
The student needs a minimum average of 86 to earn an A:
[tex]\dfrac{425+x}6=86\implies425+x=516\implies x=91[/tex]
so the student needs a score of 91.
