I'm guessing the 20% is for a big exam, so your grade of 91% is only 80% of your grade. Mulitply .8 by .91 and add .2 (the equivalent of getting 100 on your exam) and you get .928 OR 92% if your teacher rounds grades down.
To keep a 90% in your class you will have to get an 86 on your exam (.9-(.91*.8)/.2)
Comments
I'm guessing the 20% is for a big exam, so your grade of 91% is only 80% of your grade. Mulitply .8 by .91 and add .2 (the equivalent of getting 100 on your exam) and you get .928 OR 92% if your teacher rounds grades down.
To keep a 90% in your class you will have to get an 86 on your exam (.9-(.91*.8)/.2)