The most difficult maths riddles

Both notations are correct and historically Roman numerals were often written less consistently than today. The notation 'IIII' is often seen on clock faces. The reason for that may be practical, to avoid confusing the IV and VI when the clock is held upside down (although other theories also exist).

Quite surprisingly, most players answer that both answers 1/3 and 33% are correct. But 1/3 is more than 33%. (it is 33.3333333...)

There was no year number zero. So, the first century started on 1 Jan 1. Similarly, 21th century started 1 Jan 2001.