We solve problems

A six-digit phone number is given. How many seven-digit numbers are there from which one can obtain this six-digit number by deleting one digit?

Problem #PRU-34866

Problems Algebra and arithmetic Number theory. Divisibility Odd and even numbers Methods Pigeonhole principle Pigeonhole principle (other)

12–14 3.0

7 natural numbers are written around the edges of a circle. It is known that in each pair of adjacent numbers one is divisible by the other. Prove that there will be another pair of numbers that are not adjacent that share this property.

Problem #PRU-34875

Problems Geometry Plane geometry Geometrical inequalities Inequalities in a triangle A segment inside the triangle is smaller than the largest side ,Problems Polygons Hexagons Methods Pigeonhole principle Pigeonhole principle (angles and lengths) Regular polygons

12–14 2.0

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.

Problem #PRU-34904

Problems Algebra and arithmetic Word problems Solving problems by inequalities. Going through the cases. Methods Pigeonhole principle Pigeonhole principle (other)

11–14 3.0

Is it possible to transport 50 stone blocks, whose masses are equal to $370, 372, \dots, 468$ kg, from a quarry on seven 3-tonne trucks?

Problem #PRU-34905

Problems Geometry Plane geometry Area Area of a trapezium Area ratio Area ratio (other) Methods Pigeonhole principle Pigeonhole principle (finite number of poits, lines etc.) Quadrilateral Parallelogram Special cases Rectangles and squares. Criteria and properties

13–14 4.0

9 straight lines each divide a square into two quadrilaterals, with their areas having a ratio of $2 : 3$ . Prove that at least three of the nine lines pass through the same point.

Problem #PRU-34924

Problems Discrete Mathematics Game Theory

2.0

There is a counter on the chessboard. Two in turn move the counter to an adjacent on one side cell. It is forbidden to put a counter on a cell, which it has already visited. The one who can not make the next turn loses. Who wins with the right strategy?

Problem #PRU-34943

Problems Combinatorics Geometry on grid paper

3.0

The city plan is a rectangle of $5 \times 10$ cells. On the streets, a one-way traffic system is introduced: it is allowed to go only to the right and upwards. How many different routes lead from the bottom left corner to the upper right?

Problem #PRU-34945

Problems Set theory and logic Theory of algotithms Balance puzzles

2.0

27 coins are given, of which one is a fake, and it is known that a counterfeit coin is lighter than a real one. How can the counterfeit coin be found from 3 weighings on the scales without weights?

Problem #PRU-34962

Problems Methods Pigeonhole principle Pigeonhole principle (other) Combinatorics Systems of points and line segments Systems of points, lines, and line segments

12–14 2.0

Is it possible to arrange 1000 line segments in a plane so that both ends of each line segment rest strictly inside another line segment?

Problem #PRU-34968

Problems Algebra and arithmetic Number systems Decimal number system Number theory. Divisibility Division with remainders. Arithmetic of remainders Division with remainder

13–15 4.0

Prove that for any number $d$ , which is not divisible by $2$ or by $5$ , there is a number whose decimal notation contains only ones and which is divisible by $d$ .