### Homework No. 2

[Basic Control Flow, Functions]

Find the number of your Homework No.2 calculating the expression: (Faculty_Number % 21).
Your program must contain at least two functions.

Write a program that
(0) reads a set of floating-point data values from the input, and prints out the average (mean) of the data.
(1) reads a set of strings and prints out the shortest of them.
(2) reads two natural numbers and prints out its greatest common divisor.
(3) reads two natural numbers and prints out its least common multiple.
(4) reads two strings and prints out true, if the second string is a substring of the first and false, otherwise.
(5) reads a positive number and prints out all its divisors.
(6) reads a positive number n and prints out all prime numbers, less than n.
(7) reads words and prints them out in reverse order of their letters (for example "print" as "tnirp").
(8) reads positive numbers and prints out their binary representations.
(9) reads positive numbers and prints out their octal number base system representations.
(10) reads strings of ones and zeros and prints out the numbers, whose binary system representations are these strings.
(11) reads a positive integer N and prints the sum of this number and the number generated by the digits of N in backward order.
(12) reads a natural number n and prints out the sum: 1 + 2.3 + 3.4.5 + 4.5.6.7 +...+ n.(n + 1)...(2n - 1).
(13) reads a natural number n and prints out the sum of the prime divisors of n.
(14) reads a word and prints out "YES", if it a palindrome, i.e. a word, that reads the same backward or forward.
(15) reads two words and prints out "YES", if the second word made by transposing the letters of the first word (anagram).
(16) reads a string and prints out all repeating (at least twice) symbols in it.
(17) reads two positive integers a and b (a < b) and prints out the count of the numbers in the interval [a, b], which have no the same digits.
(18) reads a positive number n and prints out first n numbers which are divisible by 2, 3 and 5.
(19) reads two natural numbers a and b and prints out the number of digits of ab (the product of a and b).
(20) reads a positive number n and prints out a list of all perfect numbers (are equal to the sum of all of their factors), less than n.