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

In a school, there are four subjects. Seventy percent of students study English, seventy five percent of students study Science, eighty five percent of students study Mathematics and eighty percent of students study Spanish.

Can you calculate the percentage of students that study all four subjects?

Always in you, Sometimes on you;

If I surround you, I can kill you.

What am I?

Find the next number in the series below

21 32 54 87 131 ?

John went to a parrot shop in Mexico, and the parrot owner told him that his parrot is so unique that he repeats everything he hears. John got excited and immediately bought the parrot. John went home and spoke many words, but the parrot does not repeat anything.
He went again to the parrot shop and complaint to the shopkeeper, but the shopkeeper never lied. Explain?

There are four 3-link chains. All you have to do is join them into a big 12-link chain. For joining two closed links, one of the links must be cut and placed onto the other link for closing.

How many minimum links will you have to cut to make the big chain?

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

Can it be possible to build a house that has all its four walls facing towards the South?

John named his first son Alpha, second Beta, third Charlie and fourth Delta. What is the fifth son's name?

What can you hold in your right hand, but never in your left hand?