Lisa is currently taking physics as one of her electives in school. her grade at the end of the year is determined by the average of four exams, each worth 100 points. on her first test she got an 84, on her second she got a 92, and on her third an 80. what must she score on her fourth exam to get exactly 80% in the course?
a. 60
b. 64
c. 84
d. 85

  B.  64

Step-by-step explanation:

At present her total score is ...

  84 +92 +80 = 256

If "e" is the exam score Lisa must get, then for her average to be 80, e must satisfy ...

  (256 +e)/4 = 80

  256 +e = 320 . . . . multiply by 4

  e = 64 . . . . . . . . . . subtract 256

Lisa must score 64 on her last exam to have an average of exactly 80.

I prefer to work problems like this by considering the scores relative to the average. Lisa currently has (84 -80) +(92 -80) +(80 -80) = 4 + 12 + 0 = 16 points above the average she wants. Then for an average of exactly 80, she must score 16 points below that average on her last exam: 80 -16 = 64.