7 Roman Numerals

You know that 2 + 2 = 4. But do you know that ii + ii = iv?

The numbers that we normally use (1, 2, 3 etc) are called
"Arabic numerals". But we sometimes use another system for
writing numers - "Roman numerals". The Romans used letters
of the alphabet to represent numbers, and you will
occasionally see this system used for page numbers, clock
faces, dates of movies etc.

There are 7 letters used for Roman numerals:

I = 1
V = 5
X = 10
L = 50
C = 100
D = 500
M = 1000

In general, letters are placed in decreasing order of value,
eg XVI = 16. Letters can be repeated one or two times to
increase value, eg XX = 20, XXX = 30. Letters cannot be
repeated three times, so XXXX is not used for 40. In this
case, XL = 40 (50 minus 10).