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

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

3 6 12

4 8 16

5 10 ?

There are 2 people near the river and both of them want to get on the other side. The boat can only take one of them. But they got across.
How?

While handling a project, the landscaper is asked by the owner of the mansion that he wants four trees in front of his mansion that are exactly equidistant from each other.

How will he do it?

If you remove one from eleven, it becomes ten. If you remove one from nine, it becomes ten.

How is this possible?

Complete the series by replacing "?" with the correct number.

ST ND RD TH '?'

Why do you find that most of the buildings are deprived of the 13th floor?

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.

A horse was tied to a rope 5 meters long and the horses food was 15 meters away from the horse. How did the horse reach the food?

Can you replace the question mark with the correct number?