In a class in which the final course grade depends entirely on the average of four equally weighted 100-point tests, Joyce has scored 85, 83, and 86 on the first three. What range of scores on the fourth test will give Joyce a B for the semester (an average between 80 and 89, inclusive)?

66 ≤ f ≤100

Explanation

Mean= ( Σ x ) / n

Mean= sum of scores/ number of subject she took

Now, she already too 3 subject which sum is 85+83+86=254

Now we need to know range of score for her to have (grade) a mark between 80 and 89

Now let take the lower limit mean=80

The lowest score she can get is

Mean = ( Σx) / n

80=(85+83+86+f)/4

80×4= 254+f

Therefore, f= 320-254=66

Therefore the minimum score she can have to have a B is 66.

Then, let take the upper limit mean 89. i.e the maximum she can have so that she don't have an A grade.

Mean = ( Σx) / n

89=( 83+85+86+f)/4

89×4= 254+f

f= 356-254

f=102.

Therefore this shows that she cannot have an A grade in the exam. The maximum score for the exam is 100.

There the range of score is 66 ≤ f ≤100 to have a B grade

66 ≤ f ≤100 answer

Since she cannot score 102 in the examination.

you didn't post any answer choices, so its not 100% clear what your teacher is looking for

however, log(15/7) is equivalent to log(15) - log(7)

i'm using the rule log(x/y) = log(x) - log(y)