...but I'm afraid I can't recall how to answer such questions:
'After taking 3 math quizzes Elise has an average of 89. What must she score on the fourth quiz to raise her average to 91?'
Help is very much appreciated.
Let her scores in the first three quizzes be $\displaystyle x1 , x2 , x3$ respectively.
Given that $\displaystyle (x1+x2+x3)/3 = 89$
so $\displaystyle x1+x2+x3 = 267$
Now her average in four quizzes would be
$\displaystyle (x1+x2+x3+x4)/4$
x4 is her scr in the 4th quiz.
since $\displaystyle (x1+x2+x3+x4)/4=91$
$\displaystyle x1+x2+x3+x4=91(4)=364$
so $\displaystyle x4=364-(x1+x2+x3)$
Plug in the value of $\displaystyle x1+x2+x3$ from above and find x4
$\displaystyle x4=364-267=97$