Found: 8

Robinson found a chest with books and instruments after the ship wreck. Not all the books were in readable condition, but some of the books he managed to read. One sentence read “72 chickens cost *619* p”. (The starred digits were not readable). He has not tasted a chicken for quite some time, and it was pleasant to imagine a properly cooked chicken in front of him. He also was able to decipher the cost of one chicken. Can you?

Jack believes that he can place \(99\) integers in a circle such that for each pair of neighbours the ratio between the larger and smaller number is a prime. Can he be right?

Two different numbers \(x\) and \(y\) (not necessarily integers) are such that \(x^2-2000x=y^2-2000y\). Find the sum of \(x\) and \(y\).

The board has the form of a cross, which is obtained if corner boxes of a square board of \(4 \times 4\) are erased. Is it possible to go around it with the help of the knight chess piece and return to the original cell, having visited all the cells exactly once?

Prove that the triangle \(ABC\) is regular if and only if, by turning it by \(60^{\circ}\) (either clockwise or anticlockwise) with respect to point A, its vertex B moves to \(C\).

How many ways can you choose four people for four different positions, if there are nine candidates for these positions?

Let \(z_1\) and \(z_2\) be fixed points of a complex plane. Give a geometric description of the sets of all points \(z\) that satisfy the conditions:

a) \(\operatorname{arg} \frac{z - z_1}{z - z_2} = 0\);

b) \(\operatorname{arg} \frac{z_1 - z}{z - z_2} = 0\).