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There are 5 students. Each time, 2 students are weighed, giving a total of 10 readings, in kilograms, as listed below: 103, 115, 116, 117, 118, 124, 125, 
130,137,139.
What is the weight of the third heaviest student?

There are 55 students. Each time, 22 students are weighed, giving a total of 1010 readings, in kilograms, as listed below: 103103, 115115, 116116, 117117, 118118, 124124, 125125, 130,137,139 130,137,139 .\newlineWhat is the weight of the third heaviest student?

Full solution

Q. There are 55 students. Each time, 22 students are weighed, giving a total of 1010 readings, in kilograms, as listed below: 103103, 115115, 116116, 117117, 118118, 124124, 125125, 130,137,139 130,137,139 .\newlineWhat is the weight of the third heaviest student?
  1. Understand Weight Combinations: Understand that each pair of weights represents the combined weight of two students. To find the weight of individual students, we need to determine the unique weights that add up to the given totals.
  2. List Possible Pairs: List all the possible pairs that could result in the given readings. Since there are 55 students and we have 1010 readings, each student must be weighed with every other student exactly once.
  3. Identify Smallest & Largest: Notice that the smallest reading is 103kg103\,\text{kg} and the largest is 139kg139\,\text{kg}. The heaviest student must be in the pair that weighs 139kg139\,\text{kg}, and the lightest student must be in the pair that weighs 103kg103\,\text{kg}.
  4. Find Smallest Increase: To find the individual weights, we look for the smallest increase between consecutive readings. This is because the smallest increase would indicate that one student remained the same while the other student was replaced by a heavier one. The smallest increase here is from 115115 to 116116, which suggests these readings involve the same student.
  5. Deduce Correct Weight: Deduce that the student who weighs 116kg116\,\text{kg} must also be part of the 115kg115\,\text{kg} pair. Therefore, the difference between these two readings gives us the weight of one student: 116115=1kg116 - 115 = 1\,\text{kg}. However, this cannot be the weight of a student, which means we have made a mistake in our assumption. We need to find the smallest increase that involves a change of student.

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