Answer:
[tex]A_4[/tex]
Step-by-step explanation:
The cost of the e-readers and the subscription of their services are
[tex]A_1(t)=5t+350[/tex]
[tex]A_2(t)=10t+250[/tex]
[tex]A_3(t)=499[/tex]
[tex]A_4(t)=15t+179[/tex]
The functions are of the form
[tex]A(t)=mt+c[/tex]
Where [tex]m[/tex] is the monthly subscription cost and [tex]c[/tex] is the cost of the e-reader.
So, 15 dollars is the highest subscription cost which is of the 4th e-reader.
Hence [tex]A_4[/tex] has the highest subscription cost.