Answer:
11
Explanation:
You have an average of 7 on 3 tests, and you want to know the score required of the 4th test to make the average be 8.
Average
The average is the sum, divided by the number.
If you have an average of 7 on the first 3 tests, the sum of those scores is ...
average = sum/number
7 = sum/3
21 = sum
If you want an average of 8 on 4 tests, the sum of the scores must be ...
8 = sum/4
32 = sum
In order to increase the sum of scores from 21 to 32, you must have a score of ...
32 -21 = 11
You need to score 11 on the 4th test to bring the average to an 8.
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Alternate solution
The difference between the scores you have and the average you want is ... 3(7 -8) = -3.
In order to bring your average up to 8, you need to have a total of 3 more points than that average on your 4th test: 8 + 3 = 11. (This brings the total of differences from average to zero: (-1) +(-1) +(-1) +(3) = 0.)
Your 4th test must have a score of 11 to bring your average on 4 tests up to 8.