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

What word is represented by this rebus?

Solve the below puzzle by replacing the question mark with the correct number.

3 6 12

4 8 16

5 10 ?

Using the clues below, what four numbers am I thinking of?

The sum of all the numbers is 31.
One number is odd.
The highest number minus the lowest number is 7.
If you subtract the middle two numbers, it equals two.
There are no duplicate numbers.

One is to three as three is to five and five is to four and four is the magic number.
What is the pattern?

What's a single-digit number with no value?

I am a word that begins with the letter “i.” If you add the letter “a” to me, I become a new word with a different meaning, but that sounds exactly the same. What word am I?

What is a word made up of 4 letters, yet is also made up of 3. Sometimes is written with 9 letters, and then with 4. Rarely consists of 6, and never is written with 5.

Look at this sequence from top to bottom. What is the next number in the sequence?
1
11
21
1211
111221
312211

What do you see in the given picture?