Let us deal with HEXADECIMAL
it is called base 16
base 2 units : 0 1
base 10 units : 0 1 2 3 4 5 6 7 8 9
base 16 units : 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
but, unit is one symbol(edited)
so one representation of hexadecimal is like that :
0 1 2 3 4 5 6 7 8 9 A B C D E F
(stops at 15 with 10 being A)
we do it like that as we don’t have a special symbols for a number in base 16(edited)

see our last example, converting 544 base 16 to base 10
that was easy as we did not go beyond 10
now convert let us say
BA1 to base 10
B is 11
A is 10
1 well is 1
we apply the same process
BA1 start as usual with rightmost
1 × 16^0    | 1 × 1      | 1
10 × 16^1 | 10 × 16   | 160
11 × 16^2 | 11 × 256 | 2816
…………………………………………………
.                                       2977

Hexadecimal representations
hex values are represented variously
#FFFFFF in html p.s. it has nothing to do with color, it just represents a value where FF is 255. (change proposed by @WittyBit )
0x is also prefixed
like 0xFF
in C language
some use \$ before
some
H’1E’
some h after
2425h
also unicode
\u hex nums ( @Sylver )
if you know of other tell me