Math Test 1
1. The average of 9 observations was 9, that of the first 5 being 10 and that of the last 5 being 8. What was the fifth observation ?

Given: • Average of 9 numbers = 9 - Total = 9 × 9 = 81 • Average of first 5 numbers = 10 - Total = 10 × 5 = 50 • Average of last 5 numbers = 8 - Total = 8 × 5 = 40 The 5th number is counted in both the first and last 5 numbers. So: Add both totals:50 + 40 = 90 But the real total is only 81, so the 5th number was counted twice. To fix that, subtract the extra 5th number once: 90 - x = 81 → x = 9 Answer: 5th observation is 9.