The most difficult math riddles

There was no year number zero. There were only 1999 years between 1000BC and 1000AD.

For each circle, the length of the circumference is c=2πr. Hence, if a radius grows by 1 meter, the circumference grows by 2 π meters - no matter what the initial value of radius was.

The Gregorian Calendar omits 3 leap days every 400 years, omitting February 29 in the 3 century years (integer multiples of 100) that are not also integer multiples of 400. For example, 1600 was a leap year, but 1700, 1800 and 1900 were not. Similarly, 2000 was a leap year, but 2100, 2200, and 2300 will not be.

14 of March is celebrated on many schools and universities as both Pi Day and the birthday of Albert Einstein. One of the traditions is the 'Pi Pie' (pictured).

The possible codes are from 0000 to 9999 inclusive. There are 10 possibilities for each ring, thus there are 10*10*10*10 possible codes.

A prime number has no positive divisors other than 1 and itself. Hence, a prime bigger than 2 cannot be even. All prime numbers bigger than 5, written in the usual decimal system, end in 1, 3, 7, or 9.

The Möbius strip is a surface with only one side and only one boundary component (edge). A line drawn starting from the seam down the middle will meet back at the seam but at the "other side". If continued the line will meet the starting point and will be double the length of the original strip.

The stock price was 10 two days ago. Yesterday it dropped by 50% to 5. Today it raised by 50% (of 5) to 7.5

A cube has six square faces, each of which has four edges - but each edge is shared with another face.