When I get multiplied by any number, the sum of the figures in the product is always me. What am I?

What month of the year has 28 days?

How many letters are in the alphabet?

I am the fastest animal but cannot climb the tree. What am I?

The dirtier I get, the whiter I look. Who am I?

What has one letter and starts with an E?

In an interview, a boy was asked an unusual question 'How two persons sitting with a table in between them can't see each other?' He was unable to reply. Can you?

John enters a small room. The Door Closes.
When the door opens, John is in a larger room.

Explain?

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.

There is a five-letter word which has 3 consonants(all are same) and 2 different vowels.
Also, something wrong is associated with the word.

Can you name it?