what is the relationship between D and s?

Question

At a local store, shirts cost $12.00 for the first one and $9.00 for each additional shirt. If tara spends D dollars for s shirts, what is the relationship between D and s?

Answer 1

 

D = 12 + 9(s - 1)

 

let's prove above, let's say you only bought one shirt:

 

D = 12 + 9(1 - 1) = 12 + 9(0) = 12 , since only one shirt was bought only $12 was craged.

 

let's say you bought two shirts:

 

D = 12 + 9(2 - 1) = 12 + 9(1) = 21, since the second shirt cost $9 dollars total would be 21. So everything checks out.