1a) 13 = 5 + 2 + 2 + 2 + 2
b) 16 = 5 + 5 + 2 + 2 + 2
c) 21 = 5 + 5 + 5 + 2 + 2 + 2
The important part of the example in the textbook is the part in which they show how to get from n¢ to (n + 1)¢. Make sure you have read and understood this part of the example.