There are 25 children in a class. At random, two are chosen. The probability that both children will be boys is 3/25. How many girls are in the class?
A 3×4 rectangle contains 6 points. Prove that amongst them there will be two points, such that the distance between them is no greater than $\sqrt5$
There are 7 points placed inside a regular hexagon of side length 1 unit. Prove that among the points there are two which are less than 1 unit apart.
51 points were thrown into a square of side 1m. Prove that it is possible to cover some set of 3 points with a square of side 20cm.
Prove that it is not possible to completely cover an equilateral triangle with two smaller equilateral triangles.