Find The Next Number In The Sequence

30 10 15 13 0 16 -15 ?

I am a type of word that is spelt incorrectly or used differently, on purpose for humour. What am I?
Hint 1: You can find me in a joke.
Hint 2: I am a play on words.

Find the Next Number in the Sequence

2 9 3 1 8 4 3 6 5 7 ?

If nine thousand, nine hundred nine dollars is written as $9,909,
how should twelve thousand, twelve hundred twelve dollars be written?

Find the next number in the series below

21 32 54 87 131 ?

The centre of the third figure is empty. What digit should fit inside?

Sherlock Holmes was challenged to make a pre-drawn line smaller without erasing it. He did it in fractions of seconds. How?

Can you find a five letter word that is left with only two letters if one is removed?

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

John was writing his first book. After saving the document, he locked his laptop with a password and mentioned some phrases for the hint box.

A friend of his tried opening his laptop but found out that it was password protected. Following is the hint that appeared.
1 mobile 3 books 2 roars 1 night 4 balls 2 lighters 1 ghost 1 hat 3 watches.

Can you help him in cracking the password?