When the cost of a notebook drops by 25%, a customer is able to purchase 5 additional notebooks for Rs. 300. What is the discounted price (in Rs.) of a single notebook?

The correct answer is 15.

Step 1: Understand the given problem
Total amount available for purchase = Rs. 300
Percentage decrease in price = 25%
Number of additional notebooks bought = 5

Step 2: Direct solution using the savings method
When the price of each notebook drops by 25%, the total money saved out of Rs. 300 is equal to 25% of Rs. 300:

$\text{Savings}=25\%\text{of}300$

$\text{Savings}=\frac{25}{100}\times 300=\text{Rs.}75$

This saved amount of Rs. 75 enables the customer to buy 5 extra notebooks at the new (discounted) rate.
Therefore, the discounted price of a single notebook is:

$\text{Discounted Price}=\frac{75}{5}=\text{Rs.}15$

Step 3: Alternative verification using algebra
Let the original price of a single notebook be $P$ rupees.
The number of notebooks purchased at the original price for Rs. 300 is:

$N_{\text{original}}=\frac{300}{P}$

The discounted price per notebook is 25% less than original price, which is $0.75P$.
The number of notebooks purchased at the discounted price for Rs. 300 is:

$N_{\text{discounted}}=\frac{300}{0.75P}=\frac{400}{P}$

According to the question, the customer gets 5 more notebooks at the reduced price:

$\frac{400}{P}-\frac{300}{P}=5$

$\frac{100}{P}=5$

$P=\frac{100}{5}=20$

The original price per notebook is Rs. 20.
The discounted price per notebook is:

$\text{Discounted Price}=20\times (1-0.25)=20\times 0.75=\text{Rs.}15$

Thus, the discounted price of a single notebook is Rs. 15.