What has no start, end, or centre?

Can you replace the question mark with the correct number?

In front of you, there are 9 coins. They all look absolutely identical, but one of the coins is fake. However, you know that the fake coin is lighter than the rest, and in front of you is a balance scale. What is the least number of weightings you can use to find the counterfeit coin?

You stand in front of two doors. A guard stands next to each door. You know the following things: one path leads to paradise, the other leads to death. You cannot distinguish between the two doors. You also know that one of the two guards always tells the truth and the other always lies. You have permission to ask one guard one question to discover which door leads to paradise. What one question would you ask to guarantee you enter the door to paradise?

There are 2 sand hourglasses.

The small one can measure 5 hours and the large one can measure 7 hours.

How can we measure 16 hours with 2 sand hourglasses running together ?

Can you solve this amazing hard number series problem?
7, 8, 10, 12, 16, 18 ?

You walk into a room that contains a match, a kerosene lamp, a candle and a fireplace. What would you light first?

What is the below Rebus indicate?

What comes
Once in a Year
Twice in a Month
4 times in a Week
6 times in a Day

Can you find the missing letter in the sequence?
S - T - I - L - ? - T - F - Y - C