Please I need help with it.

12. How many different 7 digit phone numbers are there, considering that a phone number cannot begin with \"0\"? 13. You have 20 songs on iTunes and you want to make a mix of 8 songs. How many different ways can you arrange the songs? 14. If 8 boys and 2 girls must stand in line for a picture, how many ways can they form a line if, a) There are no restrictions b) The girls want to stand in the middle c) The girls don't want to stand beside each other