If I remove one from eleven it becomes Ten and if I remove one from nine also becomes Ten. How?

Count The number of F's in the paragraph below:

"FINISHED FILES ARE THE RESULT OF YEARS OF SCIENTIFIC STUDY COMBINED WITH THE EXPERIENCE OF YEARS"

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

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?

What is more useful when it is broken?

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

Can you spot what's wrong with the following sentence?

PETER WAS ANGRY AT
HIMSELF BECAUSE HE
ACCIDENTALLY LEFT THE
THE KEYS TO HIS CAR
AT HIS FRIEND'S HOUSE.

You knock a Table over in the library, and suddenly 20 eyes are looking at you.

How many times can you subtract the number two from the number fifty?

There are a number of books on a shelf. If one book is the 4th from the left and 6th from the right, how many books are on the shelf?