On his first three tests, Jacob scored 63, 82, and 85. What score must he get on his fourth test to have an average (mean) of 80 for all four tests

Question

I am having difficulty with this problem.

Answer 1

 

(63 + 82 + 85 + x)/4 = 80

 

Can you solve for x and finish from here?

Answer 2

There is another way to monitor over/under as grades are added:

 

         Grade:    63&nbs;      82      85      x

Ave Desired:     80      80       80     80

 Over/under:    -17     +2       +5      y

 

Jacob is short 17 points, is ahead 2 points, and ahead 5 points, for a total so far of short 10 points.

 

The x must make up that difference.  Thus, x=90 is required to have a four-quiz average of 80. (note: y = +10)

Answer 3

63 + 82 + 85 + x = 80

            4

 

230 + x = (80)(4)

 

230 + x = 320

 

x = 90