12. How many ways can 5 sopranos and 4 altos be selected from 7 sopranos and 9 altos? 13. How many different ways can 8 speakers be seated on a stage? 14. A soda machine servicer must restock and collect money from 15 machines, each one at a different location. How many ways can she select 4 machines to service in one day? 15. One company's ID cards consist of S letters followed by 2 digits. How many cards can be made if (a) repetitions are allowed (b) repetitions are not allowed 16. How many different arrangements of the letters in the word number can be made? 17. A physics test consists of 25 true/false questions. How many different possible answer keys can be made