So far, a student has earned the following scores on four 100-point tests this grading period:65,73,81, and 82. What score must the student earn on the fifth and last 100-point test of the grading period to earn an average test grade of 80 for the 5 tests?
A. 75
B. 76
C. 78
D. 99
E. The student cannot earn an average of 80.
解析：The correct answer is D. To find what the student needs to score on the fifth 100-point test to average a score of 80, you need to find the point total for the student so far by adding 65,73, 81 and 82. That sum is 301. Averaging 80 points on 5 tests means the student must earn 400 points (80·5). The acore needed on the last test is the difference,400～301, or 99.
正确答案选D。该学生前四次共得301分；5次考试平均分为80，意味着5次考试总分需达到400分，那么该学生最后一次考试需要达到的分数为400与301的差，即99分。
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