Find The Next Number In The Sequence

30 10 15 13 0 16 -15 ?

It can't be seen, can't be felt, can't be heard, and can't be smelt.
It lies behind stars and under hills, And empty holes it fills.
It comes first and follows after, Ends life, and kills laughter.
What is it?

How can you drop a raw egg from a height onto a concrete floor without cracking it?

A pond had 60 fish, out of which, all but 20 died.
How many fishes are now present in that pond?

How many letters are in the alphabet?

Solve the following series:
AZ, GT, MN, __, YB

I look flat, but I am deep. Hidden realms I shelter. Lives I take, but the food I offer. At times I am beautiful. I can be calm, angry, and turbulent. I have no heart but offer pleasure as well as death. No man can own me, yet I encompass what all men must have. What am I?

What has six faces, but does not wear makeup, has twenty-one eyes, but cannot see?

In the Chess Board picture below white army is arranged. You need to add a black army on the board such that no piece is under any threat.
Note: Army comprised of 1 king, 1 queen, 2 rooks, 2 bishops, 2 knights, and 8 pawns.

What is the difference between a jeweller and a jailer?