Is octaland hexadecimal numbering are the number system that is used by computers?

Answer 1

Computers actually use binary number systems, since a switch can either be on or off.
However, contents of computer memory is more concisely represented by octal and hexadecimal representations because
1. they are easy to convert to and from binary system
2. they are condensed, and exhibits patterns much better than long chains of binary numbers.

So in summary, even though octal and hexadecimal numbers are not used directly by computers, they are closely related to the binary system, and are extensively used in computer related literature.