Final answer:
By setting up an equation using the given information and solving for b, the cost of the backpack, we find that the backpack cost $35.
Step-by-step explanation:
To solve for the cost of the backpack, we can set up an equation based on the information given. Let's let b represent the cost of the backpack. According to the problem, the cost of the book is $20 less than twice the cost of the backpack, so we can express this as:
Book cost = 2b - $20
Since we know Julia spent $50 on the book, we can set up the following equation:
$50 = 2b - $20
Adding $20 to both sides gives us:
$70 = 2b
Dividing both sides by 2, we find the cost of the backpack:
$35 = b
Therefore, the backpack cost $35.