Can you find out which number multiplied by itself will give the output as 12345678987654321?

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.

You are given four results as below:
1111 = F
2222 = E
3333 = T
Then, can you find out how to code 4444?

What does the below rebus riddle means?

A 52% bias toss for head using the 51% tail bias coin was done to obtain a fair result.

Can you find how bias is the floor in this case?

If 5 cats catch 5 mice in 5 minutes, how long will it take one cat to catch a mouse?

A maths symbol is hidden in the below Bar Graph. Can you decipher it?

If you go to the movies and you're paying, is it cheaper to take one friend to the movies twice, or two friends to the movies at the same time?

Find The Next Number in this series.
1, 2, 6, 42, 1806 ?

Can you find a pattern and the missing number in the series given below?

100, 155, 258,? , 584, 819.