The Core Percentage Formula
The most basic percentage calculation is: Percentage of a number = (Percentage ÷ 100) × Number. For example, 20% of ₹1,500 is (20 ÷ 100) × 1,500 = ₹300. Almost every everyday percentage problem is a variation of this one formula.
Calculating a Discount
To find a discounted price, first calculate the discount amount using the formula above, then subtract it from the original price. For a ₹2,000 item with a 15% discount: discount = (15 ÷ 100) × 2,000 = ₹300, so the final price is ₹2,000 − ₹300 = ₹1,700. For multiple successive discounts (like ‘20% off, then an extra 10% off’), apply them one after another on the reduced price each time — they don’t simply add up to 30% off the original price.
Calculating a Tip
A tip calculation uses the same core formula: Tip = (Tip percentage ÷ 100) × Bill amount. For a ₹800 bill with a 10% tip: tip = (10 ÷ 100) × 800 = ₹80, making the total payable ₹880. Some people prefer calculating on the bill before tax, others on the total including tax — either is a personal/local convention rather than a fixed rule.
Calculating Marks or Scores as a Percentage
To express a score as a percentage: Percentage = (Marks obtained ÷ Total marks) × 100. For example, scoring 68 out of 80 gives (68 ÷ 80) × 100 = 85%. This is the reverse direction from the discount/tip examples — here you’re finding what percentage one number is of another, rather than finding a percentage of a number.
Percentage Increase or Decrease
To find a percentage change between two values: Percentage change = [(New value − Old value) ÷ Old value] × 100. A positive result is a percentage increase, a negative result is a percentage decrease. This pattern shows up in tracking price changes, salary hikes, or comparing performance between two periods.