Maths Olympiad Prep

Library / /39 of 45

, 2008

Algebra Difficulty 6.3 National olympiad Prove it Slovenia

At the National Mathematical Olympiad the students were given 4 problems. Each solution was awarded with an integral number of points between 0 and 7. There were 42 participants and exactly half of them achieved at least 50% of the points. To win the award one had to get at least 22 points and one sixth of the contestants achieved this. Together, the students who did not get the award had three times as many points as those who did. Prove that there were at least 6 contestants such that each of them got at least 25 points but none of them got more than 50%.

Solution

First note that 7 contestants won the award (one sixth of 42). The upper half consisted of 21 contestants, so 217=1421 - 7 = 14 got between 14 and 21 points.
Each of the 7 contestants who won the award had to get at least 22 points, so together they had at least 227=15422 \cdot 7 = 154 points.
Let xx represent the number of contestants who got at least 25% but less than 50% of the points, that is at least 7 but not more than 13 points. The

remaining 4221x=21x42 - 21 - x = 21 - x contestants achieved less than 6 points.
What was the total number of points given to those that did not win the award? For those with at most 6 points the total was at most 6(21x)6 \cdot (21 - x), and for those with at least 7 but less than 13 points the total was at most 13x13x points. Finally, we have to consider those 14 contestants from the upper half with at most 21 points. They account for at most 2114=29421 \cdot 14 = 294 points.
This makes for at most 6(21x)+13x+294=420+7x6 \cdot (21 - x) + 13x + 294 = 420 + 7x points. On the other hand, this number should be three times as great as the number of points given to those who won the award, which is 3154=4623 \cdot 154 = 462. Thus, we have 462420+7x462 \le 420 + 7x, or, equivalently, 7x427x \ge 42 and so x6x \ge 6. We conclude there exist at least 6 contestants such that each of them got at least 25 points but none of them got more than 50%.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.