Question 668684
To get a B or an average of 80 on 15 quizzes, the sum total of your scores must equal to 80 x 15 or 1200 points total.
If your average is 78.25 on 12 quizzes, you need to find out how many points out of the 1200 total that you still need. To do this, you multiply 78.25 x 12 to get the total points you have so far.  78.25 x 12 = 939
Now to find out how many points you need on the final 3 quizzes, subtract 939 from the original 1200 points needed.  1200-939 = 261
Since you are looking for the average grade you need on the final 3 quizzes, divide 261 by 3 to get the final average grade you need on the last 3 quizzes or 87.
1200/15 = 80
939+261 = 1200
939/12 = 78.25
261/3 = 87