If eleven plus two equals one, what does nine plus five equal?

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

Seven Robbers robbed a bank and hide the coins in a lonely place.
They decide to divide the money equally the next morning. Two greedy robbers decided to cheat the others and reach the place at night. They equally divided the coins between them, one coin left. So they called another robber and then they decided to divide equally among the three. Sadly again one coin left. The same thing happened to the 4th 5th and the 6th robber.
However, when the 7th robber reached in the morning, they can divide the coins equally.

How many coins were there in total?

How many letters are in the alphabet?

There is a five-letter word which has 3 consonants(all are same) and 2 different vowels.
Also, something wrong is associated with the word.

Can you name it?

I’m light as a feather, yet the strongest person can’t hold me for five minutes. What am I?

I look flat, but I am deep. Hidden realms I shelter. Lives I take, but the food I offer. At times I am beautiful. I can be calm, angry, and turbulent. I have no heart but offer pleasure as well as death. No man can own me, yet I encompass what all men must have. What am I?

How many letters are in the alphabet?

What you cannot hold but it is yours?

The dirtier I get, the whiter I look. Who am I?