Number Systems Humans use the decimal number system to ...

Hexadecimal Subtraction
10 - 1 = F
10 - 2 = E
11 - 2 = D
Subtraction that crosses between numbers and letters can be confusing. For example,
Seeing 10-1, your automatic reaction will be 9, but try to focus on the number system you are
using.
Summary on the Use of Number Systems
You might wonder how these number systems are used. The computer can use all three,
and people have set up computers to use the most appropriate system for various purposes. For
example, generally the computer uses decimal * for input and output, since I/O is where the
computer interacts with people, and people work best with decimal. The computer uses binary
for internal representation of everything, since that is most efficient system for processing and
storage. The computer uses hexadecimal when printing out or displaying long strings of binary
numbers for people to read. For example, if a program crashes, the computer might generate a
"core dump," which is a display of the contents of memory at the time of the crash. This dump is
printed in hex to make it easier for the programmer to read.
* Actually, I/O is done in character form, but that is a subject for another time and place.