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

What's the only room from which no one can enter or leave

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 ?

What has six faces, but does not wear makeup, has twenty-one eyes, but cannot see?

For an extra income, John decided to work at a Hotel for one hour daily. The manager offers him that they will pay him $11 after every 11 days.
However, John offered a different proposition to the manager. The offers stand as:
He will be paid just a penny on his first day.
Two pence will be paid on the second day,
Four pence will be paid on the third day.
And so on till the 11th day.

Should the Hotel manager accept his offer?

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

Can you write down eight eights so that they add up to one thousand?

Can you count the number of quadrilaterals in the picture below?

I have two sand hourglasses:
1. A 7 minute one and
2. An 11-minute one.

Using just these 2 sand hourglasses, how can I measure time as 15 minutes?

Railroad Crossing, look out for the cars. Can you spell that, without any R's ?