Multiple choice

On Thursday, $20$ of the $25$ students in a chemistry class took a test and their average (arithmetic mean) was $80$. On Friday, the other $5$ students took the test and their average (arithmetic mean) was $90.$ What was the average for the entire class?

  1. $82$
  2. $83$
  3. $84$
  4. $85$
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A Correct answer
Explanation

Total sum = (20 * 80) + (5 * 90) = 1600 + 450 = 2050. Total students = 25. Average = 2050 / 25 = 82.

AI explanation

Calculate the total sum of the scores by multiplying each group's size by its average. The first group's sum is 20 multiplied by 80, giving 1600, and the second group's sum is 5 multiplied by 90, giving 450. The combined total score for the entire class is 1600 plus 450, which equals 2050. Dividing this total sum by the total number of students (25) gives the class average: 2050 divided by 25 is 82.

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