Can you find out the next digit in the following series ?

0, 0, 1, 3, 2, 6, 3, 9, 4, 12, 5, ?

I am under you when I am whole.
I shift above you if you remove the first letter.
I come all around you if you remove the second as well.

Can you tell me who I am?

How do you go from 98 to 720 using just one letter?

There is a box in which distinct numbered balls have been kept. You have to pick two balls randomly from the lot.

If someone is offering you a 2 to 1 odds that the numbers will be relatively prime, for example
If the balls you picked had the numbers 6 and 13, you lose $1.
If the balls you picked had the numbers 5 and 25, you win $2.

Will you accept that bet?

I have feet, but no legs. Which animal Am I?

If it took eight men ten hours to build a wall, how long would it take four men to build it?

Find the missing number in the image below.

I travel all around the world but always stay in the corner. What am I?

It's a protector.
It sits on a bridge.
One individual can see directly through it, while others wonder what it hides.

What is it?

What will be the best approach to finding all the prime numbers less than 75 that leave an odd reminder when we divide them with 5?