Carter is hooked on a new book series, The Galaxy. Each book in the series is the same length and chronicles a different year in the Waka Waka Galaxy. Carter has cleared off the top shelf of his bookcase to leave room for each of the books as they come out. There is a proportional relationship between the number of books on the shelf, x, and how much shelf space the books take up (in inches), y.
x (books) y (inches)
2 2
11 11
13 13
16 16
What is the constant of proportionality? Write your answer as a whole number or decimal.