We usually write numbers in decimal form (or base 10), meaning
numbers are composed using 10 different “digits”
{0,1,…,9}.{0,1,…,9}. Sometimes though it is useful to write numbers
hexadecimal or base 16. Now there are 16 distinct digits that can
be used to form numbers:
{0,1,…,9,A,B,C,D,E,F}.{0,1,…,9,A,B,C,D,E,F}. So for example, a 3
digit hexadecimal number might be 2B8.
How many 3-digit hexadecimals are there in which the first digit
is E or F?
How many 4-digit hexadecimals start with a letter (A-F)...