Can you construct a convex polyhedron, such that no two of its faces have the same number of edges?
SOLUTION
No, you can not construct such a polyhedron. Assume the opposite and consider its face, which has the largest number of sides, say k. Then the polyhedron contains at least k more faces with different numbers of sides, all less than m. However, this is clearly impossible.
Someone stole gold coins from a museum near the park. No one saw the thief take the coins, so there isn’t a description of the robber. Slylock Fox suspects one of the creatures in the park is the thief. Which one?
SOLUTION
The raccoon on the seesaw couldn’t hold the heavier bear off the ground unless he was carrying something heavy. Since gold is one of the heaviest metals, Slylock suspects the raccoon is the thief and has hidden the coins in his clothes.
Six pawns are placed in the middle squares of the main diagonal of a chess board – b7, c6, d5, e4, f3, g2. You are allowed to take any pawn on the chessboard and replace it with two pawns – one on the square above it and one on the square on its right, in case there are empty squares there. If after several moves there are no more pawns on the main diagonal, show that all the squares above it except for h8 are covered by pawns.
SOLUTION
Assign the following weights on the squares of the chessboard:
1 on the main diagonal a8 – h1
1/2 on the diagonal b8 – h2
1/4 on the diagonal c8 – h3
1/8 on the diagonal d8 – h4
1/16 on the diagonal e8 – h5
1/32 on the diagonal f8 – h6
1/64 on the diagonal g8 – h7
Every time you make the splitting move, the total sum of the numbers of the squares covered by pawns remains a constant. At the beginning that sum is 6. Since 7/2 + 6/4 + 5/8 + 4/16+ 3/32 + 2/64 = 6, all 27 squares above the main diagonal, except the top-right corner (on which you can not place a pawn in any way), must be covered by pawns at the end.
If you know that the following game has been monochromatic, i.e. no piece has moved from black to white square or vice-versa, which one is the correct position of the bishop – e3 or e4?
SOLUTION
The correct position of the bishop is e3. Otherwise, no White’s piece could have captured the last Black’s piece, moving on black squares.
You are given an 8×8 chess-board, and in each of its cells, there is written one integer. If the difference between any two adjacent numbers is -1, 0 or 1, prove that some number is repeated at least 8 times on the board.
SOLUTION
Consider the intervals spanned by the numbers in the first row, second row, third row, etc. If all of these intervals intersect each other, then there is a number, which appears in all of them. If not, there are two intervals, which are disjoint, and a number between them, which does not appear in the two rows. Now it is easy to see that this particular number will appear in every column.
While changing a tire, a motorist accidentally dropped all four wheel nuts into the sewer grate. Just when the man lost all hope to retrieve the nuts and continue on his way, a kid passed by. After hearing the story, the kid gave an advice, which enabled the driver to successfully fit a new tire and drive to the nearest service station without any more accidents. What was the advice the kid gave to the motorist?
SOLUTION
The kid suggested that the man uses one bolt from each of the other three wheels to fix the fourth one.
Ned and Jon are playing chess. Eventually, they end up in a position in which Ned (whites) is left with 2 rooks, and Jon (blacks) has just his king on the board. If Ned can mate Jon in exactly 4 different ways, what is the position of the pieces?
SOLUTION
Black king on a1, white king on e1, white rooks on c2 and h1. Ned hasn’t moved his king and rook, so he can either castle or move his king to d2, e2 or f2, resulting in a mate.
Prove that among any 9 points in (3D) space, there are three which form an obtuse angle.
SOLUTION
Let the points be labeled A1, A2, … , A9, and P be their convex hull. If we assume that all angles among the points are not obtuse, then the interiors of the bodies P + A1, P + A2, … , P + A9 should be all disjoint. That is because, for every Ai and Aj, P must be bound between the planes passing through Ai and Aj which are orthogonal to the segment AiAj. However, all of these 9 bodies have the same volume and are contained in the body P + P, which has 8 times larger volume. This is a contradiction, and therefore our assumption is wrong.
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