Answer:
The answer would be 3b + 20 = t
Explanation:
I think the question is referring to the total cost per month to be a member.
Since it costs 20$ monthly no matter what, that price will always stay the same per month.
However, it also costs $3 per book purchased. This price may not always stay the same, because you could buy a different number of books each month, and the price would be different. The cost for books depends on the number of books you buy, which is not stated in the question.
That is why you replace the number of books with a variable, in this case b.
So the equation would be 3b + 20 = t , with 3 being the cost per book, b being the number of books, 20 being the monthly cost, and t being the total amount per month.