Find the next number in the sequence

5 12 24 36 ?

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?

What 8 letter word can have a letter taken away and it still makes a word. Take another letter away and it still makes a word. Keep on doing that until you have one letter left. What is the word?

What demands an answer, but asks no questions

I have a head like a cat and feet like a cat, but I am not a cat. What am I?

Can you find out which number multiplied by itself will give the output as 12345678987654321?

A mother is 21 years older than her child. In exactly 6 years from now, the mother will be exactly 5 times as old as the child.
Where's the father?

By including only four matchsticks in below picture, you have to make "Four Triangles And Two Squares"

I am weightless, but you can see me. Put me in a bucket, and I'll make it lighter. What am I?