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?

Sherlock Holmes was challenged to make a pre-drawn line smaller without erasing it. He did it in fractions of seconds. How?

How can you leave a room with two legs and return with six legs?

If 5 cats catch 5 mice in 5 minutes, how long will it take one cat to catch a mouse?

Which English verb becomes past tense just by rearranging the letters?

A mile-long train is moving at sixty miles an hour when it reaches a mile-long tunnel. How long does it take the entire train to pass through the tunnel?

A plane crashed between the border of Mexico and America. Where do you bury the survivors?

I have lakes with no water, mountains with no stone and cities with no buildings. What am I?

The chance of Mr John winning the lottery is 10%. All participants lined up and Mr John is 4th in the row. The first three participants lose the lottery.

What is the chance of Mr John now?

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 ?