Jun 12

## 10 Dots, 10 Coins

If you have 10 dots on a table, can you always cover them with 10 pennies without overlapping?

Remark: You can assume the dots are not too close to the edge of the table.

Assume the dots lie in a plane and the radius of a penny is 1. Make an infinite grid of circles with radii 1, as shown on the picture, and place it randomly in the plane.

If we choose any point in the plane, the probability that it will end up inside some circle of the grid is equal to S(C)/S(H), where S stands for “area”. A simple calculation shows that this ratio is bigger than 90%. Therefore the probability that some chosen point in the plane will not end up inside any circle is less than 10%. If we have 10 points, the probability that neither of them will end up inside a circle is less than 100%. Therefore we can place the grid in the plane so that all dots end up in some circles. Now just place the given coins where these circles are.

Jun 11

Jun 11

## Scoring penalties

At some point in Leonel Messi’s career, the football player had less than 80% success when performing penalty kicks. Later in his career, he had more than 80% success when performing penalty kicks. Show that there was a moment in Leonel Messi’s career when he had exactly 80% success when performing penalty kicks.

Let us see that it is impossible for Messi to jump from under 80% success rate to over 80% success rate in just one attempt. Indeed, if Messi’s success rate was below 80% after N attempts, then he scored at most 4N/5 – 1/5 = (4N-1)/5 times. If his success rate was above 80% after N+1 attempts, then he scored at least 4(N+1)/5 + 1/5 = (4N-1)/5 + 6/5 times. However, Messi can not score more than one goal in a single attempt, which completes the proof.

Jun 8

## Falling Baby

How can a baby fall out of a 30-story building onto the ground and still be alive?

The baby fell from the first floor.

Jun 4

## Gun Duel

Mick, Rick, and Nick arrange a three-person gun duel. Mick hits his target 1 out of every 3 times, Rick hits his target 2 out of every 3 times, and Nick hits his target every time. If the three are taking turns shooting at each other, with Mick starting first and Rick second, what should be Mick’s strategy?

Clearly, Mick should not aim for Rick, because if he kills him, then he will be killed by Nick. Similarly, Nick should not aim for Mick, because if he kills him, then he also will be killed by Nick. Therefore, if Nick ends up against alive Mick and Rick, he will aim at Rick, because would prefer to face off a weaker opponent afterward. This means that if Rick is alive after Mick shoots, he will shoot at Nick.

Now if Mick shoots at Nick and kills him, then he will have to face off Rick with chance of survival less than 1/3. Instead, if he decides to shoot in the air, then he will face off Rick or Nick with chance of survival at least 1/3. Therefore Mick’s strategy is to keep shooting in the air, until he ends up alone against one of his opponents.

Jun 1

## Mystery Mate

White plays and mates Black in one move. However, there is a mystery in this position which has to be revealed first.

SOLUTION

The mystery is that someone has just placed one extra black pawn on the board – there are 9 in total. Also, no matter which one is the added pawn, there always exists a mate in one move.

If the extra pawn was a7 – Qb6
If the extra pawn was b7 – Kc6
If the extra pawn was c4 – Qb4
If the extra pawn was d3 – Qe4
If the extra pawn was e3 – Bxf2
If the extra pawn was f7 – Ke6
If the extra pawn was g6 – Rg4
If the extra pawn was h3 – Rh4