Ever came across stuff like #000000 and 0x19?
Thats what we call Hexadecimal and it works a little different from what we
use in daily life (decimal). We also got binary, what the computers work with.

Decimal:
0 1 2 3 4 5 6 7 8 9
 - 10 base symbols
 
Hex
0 1 2 3 4 5 6 7 8 9 A B C D E F
 - 16 base symbols
 
Binary
0 1
 - 2 base symbols, either ON or OFF, or YES or NO etc..
 
 
Calculating with this is pretty easy, decimal is based on powers of 10, so is
HEX based on powers of 16 and binary based on powers of 2 (wich is the number of base symbols they have).

If you have a decimal number like 150, you would calculate it like:
0 x 10^0 = 0   [ 0 x 1   ]
5 x 10^1 = 50  [ 5 x 10  ]
1 x 10^2 = 100 [ 1 x 100 ]
---------------------------- +
  150
  

And it works just as easy for Hex and binary:

HEX 0x19 (thats 25 in decimal)

9 x 16^0 = 9   [ 9 x 1  ]
1 x 16^1 = 16  [ 1 x 16 ]
--------------------------+
 25
 
 0101 1001 in binary = 89 in decimal
 
1 x 2^0 = 1
0 x 2^1 = 0
0 x 2^2 = 0
1 x 2^3 = 8
1 x 2^4 = 16
0 x 2^5 = 0
1 x 2^6 = 64
0 x 2^7 = 0
-------------------------+
  89