We have that Clarence sells yearly subscriptions to a particular magazine.
He sells at least 10 and not more than 25 subscriptions each week.
The function f(t) = 48t represents the amount of money earned for selling t subscriptions each week.
So;
10 ≤ t ≤ 25
f(t) therefore is 48(10) ≤ f(t) ≤ 48(25)
This gives: 480 ≤ f(t) ≤ 1200
So the amount of money earned f(t) for selling t subscriptions each week is all multiples of 48 between 480 and 1200, inclusive.