Single correct option
The average of 15 numbers is 60. The average of the first eight numbers is 55 and the average of the last eight numbers is 65. What is the eighth number?
- A. 55
- B. 60
- C. 65
- D. 50
Show answer and solution
Answer: (B) 60
The eighth number is counted in both the 'first eight' and the 'last eight'.
Sum of first eight = 8 × 55 = 440; sum of last eight = 8 × 65 = 520; sum of all fifteen = 15 × 60 = 900.
440 + 520 = 960 counts every number once, except the eighth, which is counted twice. So the eighth number = 960 − 900 = 60.
Sum of first eight = 8 × 55 = 440; sum of last eight = 8 × 65 = 520; sum of all fifteen = 15 × 60 = 900.
440 + 520 = 960 counts every number once, except the eighth, which is counted twice. So the eighth number = 960 − 900 = 60.
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