16. Consider the set of digits {1,3, 4,5,7,8,9} a)If the digits cannot be repeated and if the middle digit must be an odd digit, how many three-digit numbers can be formed? b)If the digits can be repeated, how many positive numbers less than 1000 can be formed? 17. Each telephone number in a city consists of 7 digits, the first of which must be 3 or 4. a) How many telephone numbers are possible? b) How many telephone numbers start with 4 and contain the digit 5 at least once? c) How many telephone numbers have no repeated digits? 18.In a dance class, the instructor taught 15 different movements. For a class presentation, the student has to perform 4 or 5 movements in sequence. What is the number of different presentation if a) there is no restrictions? b)the first and last movements must be the same?