Alocal college bookstore paid a net price of $12,500 for textbooks for the coming semester. the publisher offered a trade discount of 20%. the publisher's original list price was

The answer is  $15,625

The original price is 100%. A local college bookstore paid $12,500 which is 80% of the original price because he got 20% discount (100% - 20% = 80%).

Now, make a proportion.
If $12,500 is 80% of the original price, how much $ is original price (100%):
$12,500 : 80% = x : 100 %
x = $12,500 · 100% ÷ 80% = $15,625