As Salem says arithmetic in hexadecimal is the same as in decimal.
Decimal is base 10, there are 10 symbols 0,1,2,3,4,5,6,7,8,9.
10 in decimal is 1 ten and 0 units, 11 is 1 ten and 1 unit etc…
Hexadecimal is exactly the same except its base is 16, there are 16 symbols...