Well, let's calculate how many points in total he got so far!
He did 8 quizzes, on average 85 points: 85*8=680 points.
He scored a number of points in his ninth quiz, let's call this number "x".
If we want to calculate the average after the ninth quiz, we take the sum of points then (which is 680+x) and we divide it by 9:
(680+x)/9
we also know that this is equal to 81:
(680+x)/9=81
[tex] \frac{(680+x)}{9}=81[/tex]
multiply both sides by 9:
[tex] (680+x)}=729[/tex]
substracting 680 from both sides:
x=49
well, i got 49 too.
I really can't see where the mistake could be?
where did you check it? my best guess is that the place where you checked it had a typo...?
