Question 268616
Going into the final exam, which will count 2/3's of the final grade, 
Mike has test scores of 86, 80, 84, 90. 
What score does Mike need on the final in order to earn a B, which requires an average score of 80?:
:
I think you can do it that way, this is the way I would do it:

Average the 4 grades that represent 1/3
{{{(86+80+84+90)/4}}} = 85
:
x = 2/3 the final grade or twice as much as first 4 grades
:
{{{(85 + 2x)/3}}} = 80
multiply by 3
85 + 2x = 3(80)
2x = 240 - 85
2x = 155
x = {{{155/2}}}
x = 77.5 to get 80% for a B
:
Your way would get the same answer. 
:
What score would he need for an A?