We usually write numbers in decimal form (or base 10), meaning numbers are composed using 10 different "digits" {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}. So for example, a 3 digit hexadecimal number might be 2B8. a. How many 4-digit hexadecimals are there in which the first digit is E or F ? b. How many 3-digit hexadecimals start with a letter (A-F) and end with a numeral (0-9)? c. How many 6-digit hexadecimals start with a letter (A-F) or end with a numeral (0-9) (or both)