Simple interest

Story hook

Imagine your elder cousin Riya wants to buy a bicycle, but she is short of money. She comes to you and says, "Lend me Rs 1000 for one year. And to say thank you, I will give you a little extra on top when I return it — say, Rs 100 extra."

So after one year Riya gives you back Rs 1100 — your original Rs 1000, plus Rs 100 as a small reward for letting her use your money.

That little reward — the Rs 100 extra — has a special name. It is called interest. It is the "rent" you charge for letting someone use your money for a while. Just like you pay rent for using a house that isn't yours, people pay interest for using money that isn't theirs.

In this lesson we will learn exactly how that extra money is worked out — how banks, shops and your cousin all decide on the "rent" for money. It comes from a small, friendly formula, and once you see it, it will feel as natural as sharing chocolates. Let's start from absolute 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 just need 33% (that is 66 marks out of 200) to pass. Simple interest is one of the easiest and most predictable topics. It is basically one small formula. Learn it well and you grab quick, safe marks.
• Almost every simple-interest question is solved by the same one line of maths. Few topics give you so much reward for so little effort.

For real life (this is the useful part):

• A bank says "we pay 6% interest per year on savings." You will know how much extra money your savings earn.
• You borrow money to buy a phone "on EMI." You will understand how much extra you are paying the lender.
• A fixed deposit (FD) — money you lock in a bank for a few years — grows by interest. You will be able to work out the final amount.
• A late fine on a library book or an electricity bill often grows in the same way. You will spot how it adds up.

So this is a true life skill — it protects your money and helps you make smart choices. And the best part: it is just one gentle formula. Stay relaxed and follow along step by step.