Problems

Age
Difficulty
Found: 1942

Find a natural number greater than one that occurs in the Pascal triangle a) more than three times; b) more than four times.

How many times greater is the sum of the numbers in the hundred and first line of the Pascal triangle than the sum of the numbers in the hundredth line?

Let’s put plus and minus signs in the 99th line of Pascal’s triangle. Between the first and second number there is a minus sign, between the second and the third there is a plus sign, between the third and the fourth there is a minus sign, then again a plus sign, and so on. Find the value of the resulting expression.

Prove that the sum of

a) any number of even numbers is even;

b) an even number of odd numbers is even;

c) an odd number of odd numbers is odd.

Prove that the product of

a) two odd numbers is odd;

b) an even number with any integer is even.

A road of length 1 km is lit with streetlights. Each streetlight illuminates a stretch of road of length 1 m. What is the maximum number of streetlights that there could be along the road, if it is known that when any single streetlight is extinguished the street will no longer be fully illuminated?

Several guests are sitting at a round table. Some of them are familiar with each other; mutually acquainted. All the acquaintances of any guest (counting himself) sit around the table at regular intervals. (For another person, these gaps may be different.) It is known that any two have at least one common acquaintance. Prove that all guests are familiar with each other.

15 MPs take part in a debate. During the debate, each one criticises exactly \(k\) of the 14 other contributors. For what minimum value of \(k\) is it possible to definitively state that there will be two MPs who have criticised one another?