Shortcut Trick
Net deviation of first 4 numbers = 4 × (44 − 42) = +8
Net deviation of last 5 numbers = 5 × (40 − 42) = −10
Sum of 5th, 6th, and 7th numbers = (3 × 42) − (8 − 10) = 126 + 2 = 128
Let 6th number be x, then (x + 6) + x + (x + 5) = 128 ⇒ 3x = 117 ⇒ x = 39
Average of 6th (39) and 7th (44) = (39 + 44) ÷ 2 = 41.5
∴ The correct option is D.
Alternate Method
Given:
Average of 12 numbers = 42
Average of first 4 numbers = 44
Average of last 5 numbers = 40
6th number is 6 less than 5th and 5 less than 7th.
Formula Used:
Sum = Average × Number of observations
First 4 (Avg 44)
5, 6, 7 (Avg ?)
Last 5 (Avg 40)
Total of 12 Numbers (Avg 42)
Remaining Sum (5th+6th+7th) = 128
Calculations:
⇒ Total sum of 12 numbers = 12 × 42 = 504
⇒ Sum of first 4 numbers = 4 × 44 = 176
⇒ Sum of last 5 numbers = 5 × 40 = 200
⇒ Sum of 5th, 6th, and 7th numbers = 504 − (176 + 200) = 504 − 376 = 128
⇒ Let 6th number = x
⇒ 5th number = x + 6
⇒ 7th number = x + 5
⇒ (x + 6) + x + (x + 5) = 128
⇒ 3x + 11 = 128 ⇒ 3x = 117 ⇒ x = 39
⇒ 6th number = 39, 7th number = 39 + 5 = 44
⇒ Average of 6th and 7th numbers = (39 + 44) ÷ 2 = 83 ÷ 2 = 41.5
∴ The correct option is D.
Additional Information
Average Concept
Average = (Sum of all observations) ÷ (Total number of observations). It represents the central value of a data set.
Deviation Method
The sum of deviations of all observations from the actual mean is always zero. This property helps in finding missing values quickly.
Weighted Average
When two or more groups are combined, the new average A = (n1a1 + n2a2) ÷ (n1 + n2), where n is the count and a is the individual group average.