in BCD (Binary coded decimal)  4 binary digits use to represent a decimal numbers 0 to 9.
Example:

0 : 000
1 : 0001
2:  0010
3:  0011
4: 0100
5: 0101
6: 0110
7: 0111
8: 1000
9: 1001

Each of the decimal number written using 4 bits ( 0010 for 2 and 0011 for 3)

A practical application of BCD found in calculators


BBCD also used in time coding of film and video
10:01:12 written as  0000:0001:00010010

Convert following numbers to BCD

BCD Calculator