Percentage increase and decrease

Story hook

Last month, your favourite chocolate bar cost Rs 50. You went to the same shop today and the shopkeeper smiled and said, "Sorry beta, the price went up by 10%."

You stand there thinking — how much more do I pay now? You quickly work out that 10% of 50 is 5 rupees. So the new price is 50 + 5 = Rs 55. The bar got dearer, and you just figured out exactly by how much.

Then your friend runs up, all excited. "The toy shop next door is giving 20% off on that cricket ball!" The ball normally costs Rs 200. So you save 20% of 200 = 40 rupees, and you pay only 200 − 40 = Rs 160. The price went down, and again you knew the new price in seconds.

That is the whole idea of this lesson. Prices go up (increase) and prices go down (decrease) all the time — and they almost always change by a percent. Once you learn the simple steps, you will never be confused at a shop, by a discount sign, or by a salary slip again. And we will also learn a small bit of magic: what happens when a price goes up and then down (or down and then up) — because the answer surprises most people. Let's start from zero.

Why this matters for UPSC

For your CSAT exam (UPSC Prelims Paper II):

• CSAT is a qualifying paper. You do not need a top score — you only need 33% (that is 66 marks out of 200) to pass. Percentage increase and decrease questions are short, common, and easy marks once you know the trick.
• Successive changes (a discount, then a tax on top) are a UPSC favourite. They look scary but become one quick multiplication.
• This idea hides inside many other chapters — profit and loss, discount, simple interest, population growth, data charts. Master it once and several topics suddenly feel easy.

For real life (this is the fun part):

• A shop says "25% off, then 5% GST." You will know the final price.
• Your parent gets a salary hike of 8%. You will know the new salary.
• The news says "petrol price rose 12%." You will know what it means.
• A town's population grew 5% this year. You will picture the change.

So this is not just an exam topic — it is a money-and-life skill you will use forever. And it starts as simply as a chocolate bar getting a few rupees dearer. Stay relaxed and follow along.