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Problem #PRU-79462

Problems Discrete Mathematics Combinatorics Painting problems Methods Pigeonhole principle Pigeonhole principle (other) Proof by contradiction Fun Problems Word Problems Tables and tournaments Tables and tournaments (other)

12–14 3.0

The judges of an Olympiad decided to denote each participant with a natural number in such a way that it would be possible to unambiguously reconstruct the number of points received by each participant in each task, and that from each two participants the one with the greater number would be the participant which received a higher score. Help the judges solve this problem!

Problem #PRU-86556

Problems Discrete Mathematics Algorithm Theory Game theory Game theory (other) Methods Invariants and semi-invariants Invariants

11–13 3.0

There are three piles of rocks: in the first pile there are 10 rocks, 15 in the second pile and 20 in the third pile. In this game (with two players), in one turn a player is allowed to divide one of the piles into two smaller piles. The loser is the one who cannot make a move. Which player would be the winner?

Problem #PRU-88001

Problems Methods Algebraic methods Processes and operations Discrete Mathematics Algorithm Theory Theory of algorithms (other)

11–13 3.0

A traveller rents a room in an inn for a week and offers the innkeeper a chain of seven silver links as payment – one link per day, with the condition that they will be payed everyday. The innkeeper agrees, with the condition that the traveller can only cut one of the links. How did the traveller manage to pay the innkeeper?

Problem #PRU-88031

Problems Methods Algebraic methods Processes and operations Discrete Mathematics Algorithm Theory Theory of algorithms (other)

10–13 3.0

There are 6 locked suitcases and 6 keys for them. It is not known which keys are for which suitcase. What is the smallest number of attempts do you need in order to open all the suitcases? How many attempts would you need if there are 10 suitcases and keys instead of 6?

Problem #PRU-89953

Problems Discrete Mathematics Set theory and logic Mathematical logic Mathematical logic (other) Methods Pigeonhole principle Pigeonhole principle (other)

10–12 1.0

In a vase, there is a bouquet of 7 white and blue lilac branches. It is known that 1) at least one branch is white, 2) out of any two branches, at least one is blue. How many white branches and how many blue are there in the bouquet?

Problem #PRU-97790

Problems Methods Algebraic methods Counting in two ways Algebra Exponential functions and logarithms Exponential functions and logarithms (other) Calculus Real numbers Integer and fractional parts. Archimedean property

16–18 4.0

Prove that for every natural number \(n > 1\) the equality: \[\lfloor n^{1 / 2}\rfloor + \lfloor n^{1/ 3}\rfloor + \dots + \lfloor n^{1 / n}\rfloor = \lfloor \log_{2}n\rfloor + \lfloor \log_{3}n\rfloor + \dots + \lfloor \log_{n}n\rfloor\] is satisfied.

Problem #PRU-98152

Problems Number Theory Divisibility Division with remainders. Arithmetic of remainders Division with remainder Calculus Functions of one variable. Continuity Periodicity and aperiodicity Probability and statistics Probability theory Probability theory (other) Methods Algebraic methods Proof by exhaustion

14–17 4.0

A numerical sequence is defined by the following conditions: \[a_1 = 1, \quad a_{n+1} = a_n + \lfloor \sqrt{a_n}\rfloor .\]

Prove that among the terms of this sequence there are an infinite number of complete squares.

Problem #PRU-100237

Problems Methods Invariants and semi-invariants

Louise has an \(8\times 8\) chessboard with two opposite corners removes, just like in the picture below. She also has 31 \(2\times1\) dominoes. Can she tile this board with the dominoes she has?

Problem #PRU-100239

Problems Methods Invariants and semi-invariants

There is a \(3 \times 3\) grid filled with zeros. Louise is allowed to add 1 to each small square inside any \(2\times2\) grid. Can she ever get the following table as a result of her actions?

Problem #PRU-100242

Problems Methods Invariants and semi-invariants

A rectangular floor is to be covered by \(2 \times 2\) and \(1\times4\) tiles (everything is arranged). Unfortunately one tile got smashed, but we have one more tile of the other kind available. Can we retile the floor perfectly?