Final answer:
To find the regular price of one book, set up and solve the equation 10(x - 4) = 40. The regular price of one book is $8.
Step-by-step explanation:
To find the regular price of one book, we can set up an equation based on the information given. Let's assume the regular price of one book is 'x' dollars. Since the books are on sale for $4 off, the sale price of one book would be 'x - 4' dollars. Mr. Druckman bought 10 books on sale and paid a total of $40, so we can write the equation:
10(x - 4) = 40
Simplifying the equation:
10x - 40 = 40
10x = 40 + 40
10x = 80
Dividing both sides of the equation by 10:
x = 8
Therefore, the regular price of one book is $8.