What does this mathematical rebus means ?

Can you fill the blank with the correct number?

6 30 870 756030 _____

what's the probability of getting a king or a queen from a pack of 52 cards?

If you drop a 15 kg iron bar and a 5 kg bag of cotton from a height of 50 meters which will reach the ground first?

Which wheel of car does not rotate when you turn your car right?

What does an Island and the letter T have in common?

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

Twice ten are six of us,

Six are but three of us,

Nine are but four of us;

What can we possibly be?

Would you know more of us?

Twelve are but six of us,

Five are but four, do you see?

What are we?

Look at the picture and find out which square is not linked with any of the other squares.

You have $100 with you and you have to buy 100 balls with it. 100 is the exact figure and you can't go below or above the numbers and you have to use the entire $100. If there is no kind of tax applied how many of each of the following balls will you be able to buy:

Green Balls costing $6
Yellow Balls costing $3
Black Balls costing $0.10

Now, how many of each must you buy to fulfil the condition given?