If eleven plus two equals one, what does nine plus five equal?

What is the next number in the series?

15 29 56 108 208 ?

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 have a hole in my back and legs I lack.
I live where I can’t breathe and I eat without teeth.
What am I?

While going to your grandmother's house, you counted Twenty houses on the right side. While returning back to your home, you counted Twenty houses on the left side.

How many houses are there between your home and your grandmother's home?

What's the only room from which no one can enter or leave

It comprises roots which nobody sees,
Much taller than those trees,
Up and up it goes,
Still it never grows?

How many pluses should we put between the digits of 987654321 to get a total of 99, and where?

Can you make the number 24 by utilizing the numbers 1, 3, 4 and 6? You must use one number only one time and you can use mathematical operation symbols anytime anywhere.

What kind of storm is always in a rush?