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?

What word begins and ends with an E, but has only one letter?

What is the word or phrase below?
R | E | A | D

Decode below three cipher riddles:

26 L of the A
7 D of the W
1000 Y in a M

Christina, Allison and Lena are 3 daughters of John a well-known Mathematician, When I asked John the age of their daughters. He replied "The current age of her daughters is prime. Also, the difference between their ages is also prime."

How old are the daughters?

Count the number of triangles in the given picture.

I’m tall when I’m young, and I’m short when I’m old. What am I?

John said, "If yesterday was tomorrow, today would be Friday."

When did John make this statement?

There are 2 sand hourglasses.

The small one can measure 5 hours and the large one can measure 7 hours.

How can we measure 16 hours with 2 sand hourglasses running together ?

Can you find the ages of a son, father and grandfather based on the following facts?
The sum of the ages of grandfather, father and son is 140 years.
The age of the son in months is the same as the grandfather in years.
The age of the son in days is the same as the father in weeks.