Compound interest

Story hook

Imagine your grandmother gives you Rs 100 on your birthday and says, "Keep this safe. Don't spend it. I will add 10% more every year as a little reward for saving."

So at the end of year 1, she adds 10% of 100, which is Rs 10. Now you have Rs 110. Lovely.

Now the magic question: in year 2, will she add 10% of the first 100 again (another Rs 10)? Or will she add 10% of the new amount, Rs 110 (which is Rs 11)?

If she is a kind, clever grandmother, she adds 10% of the whole Rs 110 — so she gives you Rs 11 this time, not Rs 10. Now you have Rs 121.

Did you notice? In year 2 you earned one rupee more than in year 1. Why? Because this year your reward itself earned a reward. The Rs 10 from last year stayed in your pocket and earned its own 10%.

That little extra — money earning money on top of money already earned — is the whole idea of compound interest. It looks tiny at first, but over many years it grows like a snowball rolling down a hill. Let's learn it slowly, 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 high score — you just need 33% (that is 66 marks out of 200) to pass. Compound interest questions follow a fixed recipe, so once you learn the steps they become easy, safe marks.
• It builds straight on percentages and simple interest — two topics you may already know. So you are not starting fresh; you are just adding one new twist on top.

For real life (this is the genuinely useful part):

• Your bank savings and fixed deposits (FDs) grow by compound interest. Knowing it tells you how your money really multiplies.
• Your family's home loan or bike loan EMI uses compound interest — it is why a loan ends up costing more than the price tag.
• It explains how a small monthly saving can become a big amount over 20 years, and how a town's population grows year after year.

So this is a true life skill — it is literally the maths your future salary, savings, and loans will run on. And it begins as gently as a birthday gift from grandmother. Stay relaxed and follow each step.