Count the squares in the matchstick puzzle below?

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

John speaks the truth only once a day in a week. Below are a few hints for you:

First: Days are Sunday, Monday and so on.
Second: One day he says, "I lie on Monday and Tuesday".
Third: On the next day, he says, "Today is either Thursday, Saturday or Sunday".
Fourth: On the next day, he says, "I lie on Wednesday and Friday".

Can you identify the day on which he speaks the truth?

Can you replace the X (both small and big x) with the numbers between 1 to 19 such that all the rows of 3 numbers between X (big x) sum to 23?

How far can you walk into the woods?

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.

Bay of Bengal is in which state?

What has a bottom at the top?

How will you measure 15 minutes using two hourglasses of 7 minutes and 11 minutes respectively?

You see me once in June, twice in November, and not at all in May. What am I?