13 decks of cards have been mixed. What is the minimum number of cards that must be taken out from the above-mixed cards to guarantee at least one 'four of a kind?

A high school has a strange principal. On the first day, he has his students perform an odd opening day ceremony:

There are one thousand lockers and one thousand students in the school. The principal asks the first student to go to every locker and open it. Then he has the second student go to every second locker and close it. The third goes to every third locker and, if it is closed, he opens it, and if it is open, he closes it. The fourth student does this to every fourth locker, and so on. After the process is completed with the thousandth student, how many lockers are open?

When can we add 2 to 11 and get 1 as the correct answer?

Christina has four daughters, and each of her daughters has a brother. How many children does Christina have?

why number 8549176320 is one of a kind?

If it took eight men ten hours to build a wall, how long would it take four men to build it?

Can you find the six hidden animals in the picture below?

I start and end with 500 and carry a 5 in my heart. But I require the first letter and the first number to be complete. I am the name of a King.

Who am I ?

If there are fifteen crows on a fence and the farmer shoots a third of them, how many crows are left?

What am I?