What do you answer even though it never asks you questions?

Christina has four daughters, and each of her daughters has a brother. How many children does Christina have?

There are 2 people near the river and both of them want to get on the other side. The boat can only take one of them. But they got across.
How?

A man desired to get into his work building, however he had forgotten his code.
However, he did recollect five pieces of information
* Fifth number + Third number = 14
* The fourth number is one more than the second number.
* The first number is one less than twice the second number.
* The second number and the third number equals 10.
* The sum of all five numbers is 30.

What is the code ?

Find the missing number in the sequence below:

How can you leave a room with two legs and return with six legs?

What does this mathematical rebus means ?

Baseball bat and ball cost $100. If the bat cost $99 more than the ball, what is the cost of each?

ENEMY = 2 * LINK
LINK + 2 = HELL

Then what will be TAXI + HELL? Select from the options below:
a) ROTTEN
b) DANGER
c) HEAVEN
4) MORTAL

With the help clues provided in the image, can you find the 5 digit key?