Which number should replace the question mark?

Until it is measured, not known. Yet how you miss it when it has gone. What is it?

If 21x = 79x, what is the value of x?

Can you make four (4) nines (9) equal 100?

With the help clues provided in the image, can you find the 5 digit key?

Find the number of squares in the below picture

John is 45 years older than his son Jacob. If you find similarities between their ages, both of their ages contain prime numbers as the digits. Also, John's age is the reverse of Jacob's age.

Can you find their ages?

John speaks the truth only once a day in a week. Below are a few hints for you:

First: Days are Sunday, Monday and so on.
Second: One day he says, "I lie on Monday and Tuesday".
Third: On the next day, he says, "Today is either Thursday, Saturday or Sunday".
Fourth: On the next day, he says, "I lie on Wednesday and Friday".

Can you identify the day on which he speaks the truth?

What kind of storm is always in a rush?

P is the father of Q and S is the son of R. T is the brother of P and has a daughter U. If R is the sister of P, then what is the relation between U, Q and S?