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

It has five wheels, though often think four, You cannot use it without that one more, You can put things in it, you can strap things on top, You can't find it in the market, but you can still go shop. What is it?

Which is the smallest number that you can write using all the vowels exactly once?

Find the missing letter in the last circle logically in the given below picture.

P is the father of Q and S is the son of R. T is the brother of P and has a daughter U. If R is the sister of P, then what is the relation between U, Q and S?

The following "Brain Bat" means Slow Down.
S
L
O
W

Using the same logic, can you tell us what does the following Brain Bat means ?
E
V
I
G

What is the next number in this series?

4,12,84,3612....

One day, Birbal got late for a function and Akbar was highly disappointed.
Please forgive me Jahanpanah. I arrived late because my child was crying and I had to calm him down, explained Birbal.
Does it take this long to placate a child? questioned the emperor. I feel that you have no idea about how to raise a child. I order you to act like a child and I will act like your father to show you how you should have dealt with your child. Come on! Ask me all that he asked from you.
Father, I want a cow, said Birbal.
The emperor quickly ordered one of the servants to bring a cow to the kingdom.
I want to drink its milk, now, now, now! said Birbal, imitating the voice of his child.
Akbar asked his servants to milk the cow and give to Birbal.
The servants quickly milked the cow and offered the milk to Birbal. He sipped in a little and then gave the bowl to the emperor.
As Birbal put up the next demand that was put up by his child a few hours ago, the emperor was stunned and left the room.
What did Birbal say?

It is a six-letter word.
The first four letters are me.
The second and last letters are the same.
The fourth second and last letter is payment.

Who is it?

Buy me, smell me or deliver me, I do not change.

Who am I?