Book Discount

Explanation

Given:

• Original price of math book = $85,000$
• Price of math book after discount = $28,900$
• Original price of biology book = $40,000$

We need to find the percentage of the price to be paid (price after discount) relative to the original price. Let's find this ratio.

Based on the math book price:

Simplify the fraction:

Or as a percentage:

This means the price on that day is $34\%$ of the original price.

Now, we calculate the price of the biology book using the same percentage:

So, the price of the biology book on that day is $13,600$ rupiah.