What has many keys but can’t open a single lock?

Can you find out the missing number?

? 4 5 3 5

9 5 7 10 3

The person who makes it has no need for it. He who purchases it does not use it. The person who does use it does not know they are. What is it?

If you had a pizza with crust thickness 'a' and radius 'z', what's the volume of the pizza?

What has a bottom at the top?

What do you get if you add 2 to 100 four times?

Take away my first letter, then take away my second letter. Then take away the rest of my letters, yet I remain the same. What am I?

In which direction should the missing arrow point?

You have an empty wine bottle with a cork that has been secured at the top in a normal way. There is a metal ring inside the bottle that is suspended by a string.

How can you make the metal ring drop to the bottom if you are not allowed to touch anything - not the bottle, not the cork, not the thread and not the ring?

A newspaper is supposed to have 60 pages, but pages 24 and 41 are missing.
Which other pages won't be there?