Hindu–Arabic number system:
Many years ago, Hindus and Arabs developed a number system called the Hindu–Arabic number system. It is the name given to the number system that we use today.
Roman numerals:
It is the numeral system that originated in ancient Rome. This numeral system is based on certain letters, which are given values and are used as numerals. The following are the seven number symbols used in the Roman numeral system, and their values:
I 
V 
X 
L 
C 
D 
M 
1 
5 
10 
50 
100 
500 
1000 
Seven letters of English alphabet, i.e. I, V, X, L, C, D and M, are used to represent Roman numerals. Roman numerals do not have a symbol for zero. Roman numerals are read from left to right, and are arranged from the largest to the smallest. Multiplication, division and other complex operations were difficult to perform on Roman numerals. So Arabic numerals were used. The Roman numerals for the numbers 1  15 are shown below:
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
I 
II 
III 
IV 
V 
VI 
VII 
VIII 
IX 
X 
XI 
XII 
XIII 
XIV 
XV 
We can find these roman numerals in some clocks.
Rules for Roman numerals:

In Roman numerals, a symbol is not repeated more than thrice. If a symbol is repeated, its value is added as many times as it occurs.
For example, if the letter I is repeated thrice, then its value is three.

The symbols V, L and D are never repeated.

If a symbol of smaller value is written to the right of a symbol of greater value, then its value gets added to the symbol of greater value.

For example, in case of VI, I is written to the right of V. It means that 1 should be added to 5. Hence, its value is 6.If a symbol of smaller value is written to the left of a symbol of greater value, then its value is subtracted from the symbol of greater value.
For example, in case of IV, I is written to the left of V. It means that 1 should be subtracted from 5. Hence, its value is 4.

The symbols V, L and D are never written to the left of a symbol of greater value, so V, L and D are never subtracted.
For example, we write 15 as XV and not VX.
The symbol I can be subtracted from V and X only. For example, the value of IV is four and the value of VI is six.The symbol X can be subtracted from L, M and C only. For example, X is subtracted from L to arrive at 40, which is represented by XL