Can you replace the question mark with the correct number?

In a jungle where there are no street lights or any other artificial source of lights, I notice a black snake crossing the road.
How did I get sight of the snake?

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

Can you find out which triangle will have a bigger area among the following?
1. A triangle with sides 300, 400 and 500
2. A triangle with sides 300, 400 and 700

Replace the question mark with the correct number in below Picture:

Below is the sum of symbols in each row and column in the figure. Analyzing the figure, can you find out the value of the symbols?

What has many teeth, but cannot bite?

Twenty frogs are sitting on a log floating on the surface of a river. Two of them decide to jump off into the water.

How many frogs are there on the log at this moment?

A man had five children. He had $100 with him to give to his children. He decided to start with the youngest child and then give $2 more than each younger child to his next elder child.

For example, if he gives $x to the youngest child, he will give $(x+2) to the next one, $[(x+2) + 2] to the next one and so on.

Can you find out how much did the youngest one receive?

I am a prime number.
The double of myself is equal to the square of me.

which number am I?