Final answer:
To find the cost of the fourth book, subtract the total cost of the three books from the total cost of all four books.
Step-by-step explanation:
To find the cost of the fourth book, we need to subtract the total cost of the three books from the total cost of all four books.
Let's assume the cost of the fourth book is x dollars.
If the average price of the four books is $2500, then the total cost of all four books would be 4 times $2500, which is $10000.
Since the total cost of the three books is given as $6500, we can set up the equation: $10000 - $6500 = $2500 + x.
Simplifying the equation, we get $3500 = $2500 + x.
Subtracting $2500 from both sides, we find that x = $1000.
Therefore, the cost of the fourth book is $1000.