I have a 88.4 average in my Algebra 1 class. I just got a bad grade on my test; 65.5. What is my current grade now?
-KermitPiggyBear

it depends how many tests/homeworks/whatever your 88.4 average is based on

if it's based on $n$ grades then your average is now

$avg=\dfrac{88.4 n + 65.5}{n+1}$