Can you fill the blank with the correct number?

6 30 870 756030 _____

In a mathematical quiz, you are asked the following question:

Use three 9s in a mathematical expression without dividing or multiplying to form the number one.

Can you answer the question?

If an electric train is travelling south, then which way is the smoke going?

How can you take 2 from 5 and leave 4?

Assume there are approximately 5,000,000,000 (5 billion) people on Earth. What would you estimate to be the result, if you multiply together the number of fingers on every person's left hand? (For the purposes of this exercise, thumbs count as fingers, for five fingers per hand.) If you cannot estimate the number then try to guess how long the number would be.

Until it is measured, not known. Yet how you miss it when it has gone. What is it?

As shown in the image, the nine Dogs are square fenced. By constructing just two square fences can you make sure that two Dogs cannot meet each other without crossing the fence?

I don't get anymore wetter, no matter how much rain falls on me.

Do you know of a place where the wind blows south and then suddenly shifts direction towards the north?

What can go through glass without breaking it?