Medium math riddles for kids with simple answers

There is a word and six letters it contains. Take one away and twelve is what remains. What word is it?

An infinite number of mathematicians are standing behind a bar. The first asks the barman for half a pint of beer, the second for a quarter pint, the third an eighth, and so on. How many pints of beer will the barman need to fulfill all mathematicians' wishes?

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

What is the largest number you can write with just 3 digits?

Think of a number. Double it. Add ten. Half it. Take away the number you started with. What is your number?