Quickly Solve this riddle

7 - 4 + 3 * 0 + 1 = ?

Twice ten are six of us,

Six are but three of us,

Nine are but four of us;

What can we possibly be?

Would you know more of us?

Twelve are but six of us,

Five are but four, do you see?

What are we?

John is asked to stand behind Jacob and Jacob is asked to stand behind John. Both of them feel confused about it and don't know what to do?

But it is quite possible, do you know how?

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

The more of this there is, the less you see. What is it?

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

I add 5 to 9 and get 2. The answer is correct, so what am I?

A girl is twice as old as her brother and half as old as her father. In 50 years, her brother will be half as old as his father. How old is the daughter now?

Two in a corner, one in a room, zero in a house, but one in a shelter. What is it?

How many people must be gathered together in a room, before you can be certain that there is a greater than 50/50 chance that at least two of them have the same birthday?