The centre of the third figure is empty. What digit should fit inside?

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.

What is the four-digit number in which the first digit is one-third the second, the third is the sum of the first two, and the last is three times the second?

In the following series, a set of numbers is progressing with a particular pattern. Can you deduce that pattern and find the missing set?

(2 + 6), (21 + 6), (58 + 6), (119 + 6), ___

A father's child, a mother's child, yet no one's son.

Who am I?

John is asked to stand behind Jacob and Jacob is asked to stand behind John. Both of them feel confused about it and don't know what to do?

But it is quite possible, do you know how?

These types of puzzles are known as charades. What you have to do is to find two words that are referred to in the first stanza and the second stanza and put them together to form the third word in the third stanza.

Just for example, if my first refers to 'off' and my second refers to 'ice', then my whole will be the 'office'.

My first is present - future's past -
A time in which your lot is cast.

My second is my first of space
Defining people's present place.

My whole describes a lack of site -
A place without length, breadth, or height.

If 21x = 79x, what is the value of x?

John was piloting a plane behind a car but was never able to overtake it. Why?

You board a train. The train will have to enter a tunnel soon. You are claustrophobic. Which place is the best for you to sit?