Why do you find that most of the buildings are deprived of the 13th floor?

Find the missing number in the sequence below:

A certain street contains 100 buildings. They are numbered from 1 to 100. How many 9's are used in these numbers?

Spot 5 differences in two Christmas tree below:

You have a 12 liters jug full of water. You have two empty 8 liters and 5 liters jug. How can you divide the water into two equal parts using these jugs?

You and your two friends are working in a multinational company. How can you three find out the average salary of you all without disclosing your own salary to the other two?

Can you place three balls such that the equation shown in the picture holds true?

There are two candles. Both will only burn exactly for an hour. How will you use these two candles to measure forty-five minutes?

f you were running a race, and you passed the person in 2nd place, what place would you be in now?

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.