James furiously cuts a rectangular sheet of paper with scissors. Every second he cuts a random piece by an unsystematic rectilinear cut into two parts.
a) Find the mathematical expectation of the number of sides of a polygon (made from a piece of paper) that James randomly picks up after an hour of such work.
b) Solve the same problem if at first the piece of paper had the form of an arbitrary polygon.
In the Valley of the Five Lakes there are five identical lakes, some of which are connected by streams (in the image, dotted lines denote the possible “routes” of streams). Small carps are born only in lake \(S\). While the carp is growing up, it passes exactly four times from one lake to another by some stream (the carp chooses a stream at random), and then it remains in the lake in which it ended up. Of every thousand carps, an average of 375 remain in lake \(S\), and the rest remain in lake \(B\), no one else lives in the other lakes. Determine how many streams there are in the Valley of the Five Lakes.
A regular dice is thrown many times. Find the mathematical expectation of the number of rolls made before the moment when the sum of all rolled points reaches 2010 (that is, it became no less than 2010).
The point \(O\) is randomly chosen on piece of square paper. Then the square is folded in such a way that each vertex is overlaid on the point \(O\). The figure shows one of the possible folding schemes. Find the mathematical expectation of the number of sides of the polygon that appears.
The bus has \(n\) seats, and all of the tickets are sold to \(n\) passengers. The first to enter the bus is the Scattered Scientist and, without looking at his ticket, takes a random available seat. Following this, the passengers enter one by one. If the new passenger sees that his place is free, he takes his place. If the place is occupied, then the person who gets on the bus takes the first available seat. Find the probability that the passenger who got on the bus last will take his seat according to his ticket?
A fair dice is thrown many times. It is known that at some point the total amount of points became equal to exactly 2010.
Find the mathematical expectation of the number of throws made to this point.
A fly moves from the origin only to the right or upwards along the lines of the integer grid (a monotonic wander). In each node of the net, the fly randomly selects the direction of further movement: upwards or to the right.
a) Prove that sooner or later the fly will reach the point with abscissa 2011.
b) Find the mathematical expectation of the ordinate of the fly at the moment when the fly reached the abscissa 2011.
The point \(O\), lying inside the triangle \(ABC\), is connected by segments with the vertices of the triangle. Prove that the variance of the set of angles \(AOB\), \(AOC\) and \(BOC\) is less than a) \(10\pi ^2/27\); b) \(2\pi ^2/9\).
King Arthur has two equally wise advisers – Merlin and Percival. Each of them finds the correct answer to any question with probability \(p\) or an incorrect answer, with probability \(q = 1 - p\).
If both counsellors say the same thing, the king listens to them. If they say opposite things, then the king chooses a solution by tossing a coin.
One day, Arthur thought about why he had two advisers, would one not be enough? Then the king called for his counsellors and said:
“It seems to me that the probability of making the right solutions will not decrease if I keep one adviser and listen to him. If so, I must fire one of you. If not, I’ll leave it as it is. Tell me, should I fire one of you?”.
“Who exactly are you going to fire, King Arthur?”, asked the advisers.
“If I make the solution to fire one of you, I will make a choice by tossing a coin”.
The advisers went to think about the answer. The advisors, we repeat, are equally wise, but not equally honest. Percival is very honest and will try to give the right answer, even if he faces dismissal. And Merlin, honest about everything else, in this situation decides to give such an answer with which the probability of his dismissal is as low as possible. What is the probability that Merlin will be fired?
On the skin of a Rhinoceros, its folds are vertical and horizontal. If the Rhinoceros has \(a\) vertical and \(b\) horizontal folds on the left side, and on the right side – \(c\) vertical and \(d\) horizontal folds, we will say that this is a rhinoceros in the state \((abcd)\) or just an \((abcd)\) rhinoceros.
If the Rhinoceros’ itches one of his sides against a tree in an up-down movement, and Rhinoceros has two horizontal folds on this side, then these two horizontal folds are smoothed out. If there are no two folds like this, then nothing happens.
Similarly, if the Rhinoceros itches on of his sides in a back and forth movement, and on this side, there are two vertical folds, then they are smoothed out. If there are no two folds like this, then nothing happens.
If, on some side, two folds are smoothed out, then on the other side, two new folds immediately appear: one vertical and one horizontal.
The rhinoceroses often have random sides that are itchy and need to be scratched against a tree in random directions.
At first there was a herd of Rhinoceroses in the savannah \((0221)\). Prove that after some time there were Rhinoceros of state \((2021)\) in the savannah.