Which country gets ill every time?

10 people came into a hotel with 9 rooms and each guest wanted his own room. The bellboy solved this problem.
He asked the tenth guest to wait for a little with the first guest in room number 1. So in the first room, there were two people. The bellboy took the third guest to room number 2, the fourth to number 3, ..., and the ninth guest to room number 8. Then he returned to room number 1 and took the tenth guest to room number 9, still vacant.
How can everybody have his own room?

I am white when I am dirty, and black when I am clean. What am I?

Can you decipher these quotes by Albert Einstein?
Blf xzm mvevi hloev z kilyovn lm gsv ovevo lm dsrxs rg dzh xivzgvw.

Assume there are approximately 5,000,000,000 (5 billion) people on Earth. What would you estimate to be the result, if you multiply together the number of fingers on every person's left hand? (For the purposes of this exercise, thumbs count as fingers, for five fingers per hand.) If you cannot estimate the number then try to guess how long the number would be.

Which letter of the alphabet has the most water?

Look at the picture and find out which square is not linked with any of the other squares.

I have no doors but I have keys, I have no rooms but I have space, you can enter but you can’t leave! What am I?

I shave every day, but my beard stays the same. What am I?

A pond had 60 fish, out of which, all but 20 died.
How many fishes are now present in that pond?