Determine all integer solutions of the equation \(3x - 12y = 7\).
Solve the equation with integers \(x^2 + y^2 = 4z - 1\).
Prove there are no integer solutions for the equation \(4^k - 4^l = 10^n\).
Prove there are no integer solutions for the equation \(3x^2 + 2 = y^2\).
In a group of friends, each two people have exactly five common acquaintances. Prove that the number of pairs of friends is divisible by 3.
Show that any natural number \(n\) can be uniquely represented in the form \(n = \binom{x}{1} + \binom{y}{2} + \binom{z}{3}\) where \(x, y, z\) are integers such that \(0 \leq x < y < z\), or \(0 = x = y < z\).
Find the largest value of the expression \(a + b + c + d - ab - bc - cd - da\), if each of the numbers \(a\), \(b\), \(c\) and \(d\) belongs to the interval \([0, 1]\).
Two people play a game with the following rules: one of them guesses a set of integers \((x_1, x_2, \dots , x_n)\) which are single-valued digits and can be either positive or negative. The second person is allowed to ask what is the sum \(a_1x_1 + \dots + a_nx_n\), where \((a_1, \dots ,a_n)\) is any set. What is the smallest number of questions for which the guesser recognizes the intended set?
Louise is confident that all her classmates have different number of friends. Is she right?
There are 100 cities all connected by roads. Each city has 6 roads coming in (or going out). How many roads do connect those cities?