Multiple choice

Freddy was preparing for an exam and he used to take practice mocks. His average marks were 20. He took another mock and got 25 marks, and then his average increased to 21 marks per mock. How many mocks did he attempt before the mock in which he scored 25 marks?

  1. 2

  2. 3

  3. 4

  4. 5

  5. 6
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C Correct answer
Explanation

Let n be the number of mocks before the last one. Total marks = 20n. After the new mock, total marks = 20n + 25, and the new average is 21 for n+1 mocks. So, (20n + 25) / (n + 1) = 21, which leads to 20n + 25 = 21n + 21, so n = 4.

AI explanation

Let n be the number of mocks attempted before the latest one. Using the formula for updating an average, 20n + 25 = 21(n + 1). Expanding and rearranging the terms gives 20n + 25 = 21n + 21. Subtracting 20n and 21 from both sides yields n = 4. He attempted 4 mocks before the one in which he scored 25 marks.

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