If you say my name, I will no longer exist. What am I?

O_er_t_o_

Complete the missing letters of the given word.

I am thinking of a five-digit number such that:
The first and last digits are the same, their submission is an even number and multiplication is an odd number and is equal to the fourth number. Subtract five from it and we obtain the second number. Then divide into exact halves and we get the 3rd number.

What number I am thinking of?

When you stop to look, you can always see me. But if you try to touch me, you can never feel me. Although you walk towards me, I remain the same distance from you. What am I?

Alpha is Beta's sister.

Gamma is a Beta mother.

Delta is Gamma's father.

Tango is Delta's mother.

Then,

How is Alpha related to Delta?

Can you count the number of triangles in the picture below?

Buy me, smell me or deliver me, I do not change.

Who am I?

Replace the (?) with the correct mathematical signs to make the expression equal to 59.

18 ? 12 ? 4 ? 5 = 59

Why is a river so rich ?

The person who makes it has no need for it. He who purchases it does not use it. The person who does use it does not know they are. What is it?