Introduction:
Today I'm starting to learn the Pigeonhole Principle. That is another very easy kind of math problem. Wait… I said very easy, but if we look back, I didn't understand anything about the Pigeonhole Principle. After this note, please comment: did it look easy or difficult?

The exercise:

In a drawer there are 10 red socks, 10 blue socks, and 10 green socks.
Without looking, how many socks do you need to take out to be 100% sure that you have at least two socks of the same color?

We will work on the exercise. 
And yeah, I know - some of you guys will know how to solve this. Others no. Let's say from where I am. I am in the middle. Or that was before I understood it. So let's start.

The solution:
The solution always starts with the worst situation. Here they need to take two socks with the same color, so the worst situation will be to take one sock from each color.

First extraction: We take one red sock.
Second extraction: We take one blue sock.
Third extraction: We take one green sock.

And now comes the moment with the fourth extraction. But before it, let's look at what we have so far:
We have one red, blue, and green sock.
The fourth sock can be red, blue, or green.
I hope you understand it. I don't care what the fourth sock is. When we take it, the condition is met.

Answer:
And I finally found the answer:
 

We need to take 4 socks.

And one important thing. The Pigeonhole Principle is what we used in this exercise. Here is a closer explanation for that:
We have 3 drawers and 4 socks. That means there is at least one drawer with at least two socks.

End:
That is the Pigeonhole Principle. I don't know if you felt it difficult or easy. Please comment down below how you felt. Bye!