Let x represent Holly's score on 7th math quiz.
We have been given that the mean of Holly's first six math quiz scores was 75%. This means that the average on 1st 6 math quizzes is 75.
To find the total scores on 1st 6 math quizzes, we will multiply 75 by 6 that is [tex]75\times 6 =450[/tex].
We will use average formula to find test scores on 7th math quiz.
[tex]\text{Average of 7 math quizzes}=\frac{\text{Sum of scores on 7 math quizzes}}{\text{7 quizzes}}[/tex]
[tex]77=\frac{x+450}{7}[/tex]
Let us solve for x.
[tex]77\cdot 7=\frac{x+450}{7}\cdot 7[/tex]
[tex]539=x+450[/tex]
[tex]x+450=539[/tex]
[tex]x+450-450=539-450[/tex]
[tex]x=89[/tex]
Therefore, Molly's score on the 7th math quiz was 89.
