Answer:
The regular price was $30.
Step-by-step explanation:
The bookstore sells used books at 40% discount off the price of new books. This means, after the discount, the price of a used book will be 100% - 40% = 60% of the original price.
Now, if the price of the used book is $18, we know that is is 60% of the price of the book when it was new.
Let us call
, the price of the book when it was new, then we know that
<em>(this says that 60% of the original price is $18.</em><em>)</em>
We solve for
and get

The price of the book was $30 when it was new.