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

What does this mean?

CCCCCCC

A clock loses 10 minutes each hour. If the clock is set correctly at noon, what time is it when it reads 3 PM?

I cannot talk, but I always reply when spoken to. What am I?

What is harder to catch the faster you run?

The captain of a ship was telling this interesting story: "We travelled the sea far and wide. At one time, two of my sailors were standing on opposite sides of the ship. One was looking west and the other one east. And at the same time, they could see each other clearly." How can that be possible?

Using the clues below, what four numbers am I thinking of?

The sum of all the numbers is 31.
One number is odd.
The highest number minus the lowest number is 7.
If you subtract the middle two numbers, it equals two.
There are no duplicate numbers.

What demands an answer, but asks no questions

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?

If we tell you that there is a relation between the numbers and letters in the given figure, can you analyze it and find the missing letter in the last box?