Answer:
Sally's score on the 7th test needs to be 17.5 points points more than the average of her first 6 tests.
Explanation:
Let T be the sum of the scores of Sally's first 6 tests. Her average score would be:
Avg. Score (S) =T/6
Let S stand for Sally's average score.
We are told her average is k, so we can write: S = k
Let N stand for the score of her New (7th) test. Her total score would be:
T + N [This is the sum of all 7 of her tests]
Her average would become (T+N)/7
The goal is to raise her average score by 2.5 points to (k+2.5) points.
That means (T+N)/7 = k+2.5 [Her new average is 2.5 points higher]
(T + N)/7 = k + 2.5
T+N = 7k+17.5
Note that T = 6k [her total score for the first 6 tests is her average, k, times 6]
We'll use this definition of T in the equation:
T+N = 7k+17.5
(6k)+N = 7k+17.5 [Substitute 6k for T, since T=6k]
N = 7k+17.5-6k [Rearrange]
N = 1k+17.5
This is telling Sally, and us, that her new test, N, must score 17.5 points higher than her average score, k.
The answer is 17.5 points.