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

What is the next number in this matchsticks series riddle?

A man dies of old age on his 25 birthday. How is this possible?

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

Find a 9-digit number, which you will gradually round off starting with units, then tenth, hundred etc., until you get to the last numeral, which you do not round off. The rounding alternates (up, down, up ...). After rounding off 8 times, the final number is 500000000. The original number is commensurable by 6 and 7, all the numbers from 1 to 9 are used, and after rounding four times the sum of the not-rounded numerals equals 24.

I look at you, you look at me, I raise my right, you raise your left. What am I?

I am an odd number. Take away a letter and I become even. What number am I?

I can write cow in 13 letters.

Can You?

You find yourself in a strange place guarded by two guards.One of the guard always say truth while other always lies.You don't know the identity of the two.You can ask only one question to go out from there. What should you ask?

Can you solve this amazing hard number series problem?
7, 8, 10, 12, 16, 18 ?