The Junk Seller - Cost Price and Selling Price

Cost Price ($CP$) is the money paid by a buyer to purchase an item. Imagine a shopkeeper handing over cash to a factory owner to get goods for their shop; that total amount is the $CP$.

Selling Price ($SP$) is the money received by a seller when they sell an item to a customer. Picture a customer paying a shopkeeper for a notebook; the money in the shopkeeper's hand is the $SP$.

Profit (or Gain) occurs when the Selling Price is greater than the Cost Price ($SP > CP$). Visually, think of a bar graph where the 'Sell' bar is taller than the 'Buy' bar. The difference between the heights is your profit.

Loss happens when the Selling Price is less than the Cost Price ($SP < CP$). Imagine a slide where you start high (the $CP$ you paid) and end up lower (the $SP$ you received); the amount you 'dropped' is your loss.

Calculating the total price involves multiplying the price of a single unit by the total quantity. For example, if 1 kg of brass costs $₹ 180$, then a pile of 3 kg will cost $3 \times 180$.

Unitary Method is a way to solve problems by first finding the value of one unit. If you know the price of 10 pens, you divide by 10 to find the price of 1 pen, then multiply to find the price of any other quantity.

Loan and Interest are important in trade. A Loan is money borrowed from a bank or a person. Interest is the extra money paid back along with the loan. Total amount paid back = $Loan + Interest$. Visually, the bag of money you return is bigger than the bag you borrowed.

The Box Method (or Grid Method) for multiplication is often used to calculate large totals. For $30 \times 12$, you split 12 into $10 + 2$, calculate $30 \times 10 = 300$ and $30 \times 2 = 60$, then add them to get $360$.