Replace the question mark with the correct number in the below-given picture?

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

I know a 5-digit number having a property that With a 1 after it, it is three times as large as it would be with a one before it.

Guess the number?

If in a car race, the man who came two places in front of the last man finished one ahead of the man who came fifth, how many contestants were there?

You are offered an opportunity to win a fortune. There are 100 precious black stones and 100 unworthy white stones. There are two different sacks labelled as "Heads" and "Tails". You can distribute the stones as you wish. Then a coin will be flipped. Then you will have to choose a stone from the corresponding sack. If you pick up a black stone, all the fortune of black stones is yours but if you pick up white, you get nothing.

How will you distribute the stones so that you can maximize your chances of winning?

What phrase is represented by the Rebus

Five thieves looted a bank and they ran away in a car. The bank staff informed the police and they began the search of their car in their jeep. They found them on a road and chased them eventually catching them. The light that is used to fill the number plate was broken on the thieves' car. Also, the headlights of the jeep police were not working. How were the police able to catch the thieves then?

In the given figure, move three matchsticks so that the resulting figure contains two rectangles.

If 5 cats catch 5 mice in 5 minutes, how long will it take one cat to catch a mouse?

Can you arrange four 9's and use at most 2 math symbols, to make the total 100?