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4. Find the set of all positive integers n with the property that the set can be partitioned into two sets such that the product of the numbers in one set equals the product of the numbers in the other set.
6. In a plane there are 100 points, no three of which are collinear. Consider all possible triangles having these points as vertices. Prove that no more than 70% of these triangles are acute-angled.
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