Multiple choice

The average of 9 observations was 9, that of the 1st of 5 being 10 and that of the last 5 being 8. What was the 5th observation?

  1. 6

  2. 5

  3. 7

  4. 9

  5. 8
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Sum of 9 observations = 9 * 9 = 81. Sum of first 5 = 5 * 10 = 50. Sum of last 5 = 5 * 8 = 40. The 5th observation is counted in both sets, so 5th = (50 + 40) - 81 = 90 - 81 = 9.

AI explanation

The sum of all 9 observations is 9 multiplied by 9, which equals 81. The sum of the first 5 observations is 10 multiplied by 5, giving 50. The sum of the last 5 observations is 8 multiplied by 5, giving 40. Adding the first 5 and last 5 gives 90, which is 9 more than the actual total sum of 81 because the 5th observation is counted twice. Therefore, the 5th observation is 90 minus 81, which equals 9.

Keep practicing — related questions