Shortcut Trick
Using the deviation method relative to the total average of 65 kg:
Net deviation for (A + B) = 2 × (63.5 − 65) = −3 kg.
Net deviation for (A + C) = 2 × (67.5 − 65) = +5 kg.
Since A is the common element in both subsets, its weight is calculated by adding these deviations to the original average.
Weight of A = 65 + (−3 + 5) = 65 + 2 = 67 kg.
∴ The correct answer is 67 kg.
Alternate Method
Given:
Average weight of A, B, and C = 65 kg
Average weight of A and B = 63.5 kg
Average weight of A and C = 67.5 kg
Formula Used:
Sum of values = Average × Number of values
Calculations:
⇒ Total weight of (A + B + C) = 65 × 3 = 195 kg
⇒ Total weight of (A + B) = 63.5 × 2 = 127 kg
⇒ Total weight of (A + C) = 67.5 × 2 = 135 kg
⇒ Sum of (A + B) and (A + C) = 127 + 135 = 262 kg
⇒ This sum represents (A + B + C + A). Therefore:
⇒ Weight of A = (A + B + A + C) − (A + B + C)
⇒ Weight of A = 262 − 195 = 67 kg
∴ The correct answer is 67 kg.
Additional Information
Average Concept
The Average is the sum of observations divided by the total number of observations. Formula: Avg = Σx ÷ n.
Deviation Principle
The algebraic sum of deviations of all observations from their arithmetic mean is always zero. This is used to find missing values in datasets.
Weighted Average
When combining two groups, the combined average = (n1x1 + n2x2) ÷ (n1 + n2), where n is count and x is the average.