We are assuming that the party cost is directly proportional to the number of guests.
Then, if it will cost $8 for 4 guests, the unit rate is:
[tex]c=\frac{8\text{ dollars}}{4\text{ guests}}=2\text{ dollars per guest}[/tex]Then, we can use this unit rate to calculate the cost for 9 guests:
[tex]C(9)=2\frac{\text{ dollars}}{\text{ guest}}\cdot9\text{ guests}=18\text{ dollars}[/tex]The cost for 9 guests is $18.
