Chapter 2: Hide and Seek

Numbers love to play hide-and-seek. Some hide behind a "plus", some behind a "minus". Our job is to find them!

What we'll learn

• Add and subtract 3-digit and 4-digit numbers smartly.
• Find missing numbers in number sentences.
• Use estimation to guess the answer quickly.
• Solve word problems from daily life.

The big idea

When you put two groups together, you add. When you take some away, you subtract. These two are best friends , one undoes the other.

If 25 + 13 = 38, then 38 − 13 = 25 and 38 − 25 = 13. Addition and subtraction are like a mirror pair.

Sometimes a number is hidden. For example: 47 + ? = 60. To find the missing number, we subtract: 60 − 47 = 13. The hider was 13!

We do not always need an exact answer. If a shopkeeper has 198 chocolates and sells 102, we can estimate: 200 − 100 = 100 chocolates left, roughly. The real answer is 96 , very close.

Big numbers can also be added in our head by splitting them. 348 + 256 can be done as (300 + 200) + (48 + 56) = 500 + 104 = 604.

When subtracting, we sometimes borrow. To do 503 − 247, we cannot take 7 from 3, so we borrow from the next digit. Slowly, place by place, we land on 256.

Explore with examples

1. Add 367 + 248 in your head.

Split: 300 + 200 = 500, then 67 + 48 = 115. Together: 500 + 115 = 615.

2. Subtract 824 − 379.

  8 2 4
- 3 7 9
-------
  4 4 5

Borrow as needed. The answer is 445.

3. Find the missing number: 156 + ? = 400.

Do 400 − 156 = 244. Check: 156 + 244 = 400.

4. Estimate the sum 489 + 312.

Round to nearest hundreds: 500 + 300 = 800. Actual answer is 801. Very close!

Hands-on activities

Number Hunt

Write a number between 1 and 100 on a slip and hide it. Your friend asks "Is it bigger than 50?" "Smaller than 80?" Count how many questions they need to find your number.

Shopping Game

Make pretend price tags: pen ₹15, copy ₹28, eraser ₹6. Give yourself ₹100. Buy 3 items and find out how much money you have left.

Add-the-Date

Open today's calendar. Add the date numbers in any row going across. Then add the dates going down. Do the sums match? Why or why not?

Try it yourself

1. 256 + 478 = ?
2. 1000 − 645 = ?
3. Find the missing number: 89 + ___ = 200.
4. Sita has ₹350. She buys a book for ₹125 and a pen for ₹40. How much is left?
5. Estimate: 297 + 503 (no exact answer needed).
6. A bus has 52 seats. 39 are filled. How many empty seats?
7. Add: 1234 + 2345 + 1111 = ?

Recap

• Addition and subtraction are inverse , one undoes the other.
• Split big numbers into hundreds, tens and ones to add mentally.
• Estimate first to check your answer is sensible.
• A "missing number" puzzle is just subtraction (or addition) in disguise.