Can you find out the missing number in the grid?

Use only one mathematical symbol and all numbers (0-9) to get a sum of 99

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

The square root of number 121 is "11". What is the square root of the number "12345678987654321."?

Three people enter a room and have a green or blue hat placed on their heads. They cannot see their own hat but can see the other hats.
The colour of each hat is purely random. They could all be green, blue, or any combination of green and blue.
They need to guess their own hat colour by writing it on a piece of paper, or they can write 'pass'.
They cannot communicate with each other in any way once the game starts. But they can have a strategy meeting before the game.
If at least one of them guesses correctly they win $10,000 each, but if anyone guesses incorrectly they all get nothing.
What is the best strategy?

There are five vowels (a, e, i, o, u) in the English language.
Can you tell us a word that contains all these vowels?

I am the beginning of everything, the end of everywhere. I'm the beginning of eternity, the end of time and space. What am I?

A man who was outside in the rain without an umbrella or hat didn’t get a single hair on his head wet. Why?

When you say goodbye, I am what you need to tie.
I come as a couple and go wherever you run.
Who am I?

Make the seating arrangement of 10 children in such a way that there are 5 rows with 4 children in each row.