The dragon locked six dwarves in the cave and said, "I have seven caps of the seven colors of the rainbow. Tomorrow morning I will blindfold you and put a cap on each of you, and hide one cap. Then I’ll take off the blindfolds, and you can see the caps on the heads of others, but not your own and I won’t let you talk any more. After that, everyone will secretly tell me the color of the hidden cap. If at least three of you guess right, I’ll let you all go. If less than three guess correctly, I’ll eat you all for lunch." How can dwarves agree in advance to act in order to be saved?
Do there exist two numbers such that their sum, quotient and product would be all equal to each other?
It is easy to construct one equilateral triangle using three identical matches. Is it possible to construct four equilateral triangles by adding just three more matches identical to the original ones?
Winnie the Pooh has five friends, each of whom has pots of honey in their house: Tigger has \(1\) pot, Piglet has \(2\), Owl has \(3\), Eeyore has \(4\), and Rabbit has \(5\). Winnie the Pooh comes to visit each friend in turn, eats one pot of honey and takes the other pots with him. He came into the last house carrying \(10\) pots of honey. Whose house could Pooh have visited last?
Find the largest possible number of bishops that can be placed on the \(8 \times 8\) chessboard so that no two bishops threaten each other.
Does there exist a power of \(3\) that ends in \(0001\)?
There are \(24\) children in a class. Some pairs of children are friends. The friendship relation satisfies the following rules:
If someone (say Alice) is a friend of someone else (say Bob), then Bob is a friend of Alice.
If Alice is a friend of Bob and Bob is a friend of Claire, then Alice is also a friend of Claire.
Therefore Alice must be friends with herself. Is this reasoning correct?
I am going to convince you that all people have the same eye color! How? Well, notice that if there were only one person in the world, then my claim would be true. Now we will explain that if the claim is true when there are \(n\) people in the world, then it will also be true when there are \(n+1\) people. Therefore, it will be true regardless of the amount of people! (This kind of proof is called a proof by induction)
Let’s imagine that the claim is true when there are \(n\) people in the world. Now take any group of \(n+1\) people, and label them \(a_1,a_2,\dots,a_{n+1}\). Remove \(a_1\). The remaining people \(a_2,a_3,\cdots,a_{n+1}\) form a group of \(n\) people, so they must all have the same eye colour.
On the other hand, let’s remove \(a_{n+1}\). The remaining people \(a_1,a_2,\dots,a_{n}\) also form a group of \(n\) people, so they must again all have the same eye colour.
Since these two smaller groups overlap (they both contain \(a_2,a_3,\dots,a_{n-1}\)), everyone in the full group \(a_1,a_2,\dots,a_n\) has the same eye colour.
We are going to show that \(1\) is
the largest natural number.
Proof: Let \(n\) be the largest natural
number. We will show that \(n=1\).
Since \(n\) is the largest natural
number, \(n^2\), which is also a
natural number, must be less than or equal to \(n\). Therefore \(n^2-n\leq 0\). But \(n^2-n=n(n-1)\). If \(n(n-1)\leq 0\), it must be that \(0\leq n\leq 1\), and so \(n=1\).
Theorem: If we mark \(n\) points on
a circle and connect each point to every other point by a straight line,
the lines divide the interior of the circle is into is \(2n-1\) regions.
"Proof": First, let’s have a look at the smallest natural numbers.
When \(n=1\) there is one region (the whole disc).
When \(n=2\) there are two regions (two half-discs).
When \(n=3\) there are \(4\) regions (three lune-like regions and one triangle in the middle).
When \(n=4\) there are \(8\) regions, and if you’re still not convinced then try \(n=5\) and you’ll find \(16\) regions if you count carefully.
Our proof in general will be by induction on \(n\). Assuming the theorem is true for \(n\) points, consider a circle with \(n+1\) points on it. Connecting \(n\) of them together in pairs produces \(2n-1\) regions in the disc, and then connecting the remaining point to all the others will divide the previous regions into two parts, thereby giving us \(2\times (2n-1)=2n\) regions.