Transform the word THINK into BRAIN while changing only one letter at a time in a manner that each of the word in the process is a real word!!!

I am white when I am dirty, and black when I am clean. What am I?

If,
29 - 1 = 30
9 - 1 = 10
14 - 1 = 15

Based on similar logic, Can you prove that the below algebraic equation is true?
11 - 1 = 10 ?

I cannot talk, but I always reply when spoken to. What am I?

A man died and leaves Rs.10,000 in his will. There are 6 beneficiaries- his 3 sons and their wives. The 3 wives receive Rs.3960 of which Priyanka gets Rs.100 more than Tanu and Neha gets Rs.100 more than Priyanka.
Pramod gets twice as much as his wife, Tushar gets the same as his wife, and Prashant gets 50% more than his wife.

Who is married to whom?

Before the start of the car race, John and Jacob have the same amount of fuel in their car. With this fuel, John can drive for 4 hours while Jacob can drive five hours.
After a time they realize that the amount of fuel left in John's car is 1/4th of the fuel in Jacob's.

For how long they are racing?

I am under you when I am whole.
I shift above you if you remove the first letter.
I come all around you if you remove the second as well.

Can you tell me who I am?

Can you name some countries which do not have the alphabet 'A' in them?

I am soft and cuddly that soothe your heart. But, If you pick my last name I am going to tear you all apart. Who am I?

What number do you get when you multiply all of the numbers on a telephone's number pad?