In an aptitude question on averages, the average weight of Somdev, Gurdeep and Ritu is 85 kg. If the average weight of Somdev and Gurdeep is 79 kg and the average weight of Gurdeep and Ritu is 73 kg, what is the weight of Gurdeep in kilograms?

Introduction / Context:
This quantitative aptitude question tests the concept of averages and how to form and solve simple linear equations from average information. Three friends, Somdev, Gurdeep and Ritu, have a known overall average weight and you are also given the averages of two person groups. From this overlapping data, you must isolate and compute the individual weight of Gurdeep.

Given Data / Assumptions:
- Average weight of Somdev, Gurdeep and Ritu together is 85 kg.
- Average weight of Somdev and Gurdeep is 79 kg.
- Average weight of Gurdeep and Ritu is 73 kg.
- All weights are measured in kilograms.
- We need to find the weight of Gurdeep only.

Concept / Approach:
The key idea is that average multiplied by the number of persons gives the total weight. By converting each average into a total and introducing variables for the unknown weights, we can form a small system of linear equations. Solving this system step by step will give us the individual weight of Gurdeep. This method is standard in aptitude exams whenever overlapping group averages are given.

Step-by-Step Solution:
Step 1: Let the weights of Somdev, Gurdeep and Ritu be S, G and R kg respectively.Step 2: From the first statement, (S + G + R) / 3 = 85, so S + G + R = 3 * 85 = 255.Step 3: From the second statement, (S + G) / 2 = 79, so S + G = 2 * 79 = 158.Step 4: From the third statement, (G + R) / 2 = 73, so G + R = 2 * 73 = 146.Step 5: Use S + G + R = 255 and S + G = 158 to get R = 255 - 158 = 97.Step 6: Now substitute R into G + R = 146, so G + 97 = 146 which gives G = 146 - 97 = 49.Step 7: Therefore, Gurdeep has a weight of 49 kg.

Verification / Alternative check:
Check the averages using S + G = 158 and R = 97. Then S + G + R = 158 + 97 = 255 and 255 / 3 = 85, which matches the given average of 85 kg. Next, G + R = 49 + 97 = 146 and 146 / 2 = 73, which matches the given average for Gurdeep and Ritu. Finally, S + G = 158 so their average is 158 / 2 = 79, matching the second given condition. All statements are satisfied, confirming that Gurdeep must weigh 49 kg.

Why Other Options Are Wrong:
Weights such as 78 kg, 72 kg or 90 kg do not satisfy all three average relations when substituted into the equations. With those values, at least one of the calculated group averages differs from the values provided in the question. Therefore, they cannot represent Gurdeep weight in this scenario.

Common Pitfalls:
Some learners try to average the given averages directly, which does not work for overlapping groups. Others forget to convert averages into totals or miscalculate the multiplication by 2 or 3. Writing clean equations for S + G + R, S + G and G + R and carefully substituting is the safest way to reach the correct solution.

Final Answer:
The weight of Gurdeep is 49 kg.