Last week len spent $18 to bowl 4 games. this week he spent $27 to bowl 6 games. len owns his bowling ball and shoes, so he only has to pay for each game that he bowls. if each of these bowling games cost the same amount of money, what is the constant of proportionality between the money spent and the number of games played?

a. 1.5
b. 2.0
c. 4.5
d. 9.0