Money - Addition and Subtraction of Money

Understanding Currency: Indian money consists of Rupees ($\text{₹}$) and Paise ($p$). Visually, you can imagine Rupees as paper notes or large coins and Paise as much smaller coins. There are $100$ Paise in $1$ Rupee.

Decimal Notation: We use a decimal point (.) to separate Rupees from Paise. The numbers on the left of the dot are Rupees, and the two digits on the right are Paise. For example, $\text{₹}10.50$ shows $10$ in the Rupee column and $50$ in the Paise column.

The Two-Digit Rule for Paise: Paise must always be written as a two-digit number after the decimal point. If you have $5$ paise, it is written as $.05$, not $.5$. Visually, think of two empty slots after the dot that must always be filled.

Aligning Decimal Points: When adding or subtracting, the most important step is to arrange the numbers vertically so that the decimal points are in a straight line. This ensures you are adding Rupees to Rupees and Paise to Paise, similar to how you align place value columns in standard addition.

Addition with Carrying: If the sum of the Paise column is $100$ or more, you 'carry over' to the Rupees column. For example, if Paise add up to $120$, you write $20$ in the Paise column and carry $1$ to the Rupees column because $100p = \text{₹}1$.

Subtraction with Borrowing: If the Paise in the top number are less than the Paise in the bottom number, you must borrow $\text{₹}1$ from the Rupees column. This $1$ Rupee becomes $100$ Paise when moved to the Paise side. Visually, this is like 'breaking' a one-rupee note into $100$ smaller coins to help with the subtraction.

Total and Change: In word problems, 'Total' usually implies addition (joining amounts), while 'Change' or 'Difference' implies subtraction (finding what is left over after a purchase).