Chapter 9: House of Hundreds – II

Last time we filled the three rooms of the House of Hundreds. Now the house is getting fuller. Numbers are knocking on the door of 1000. What happens when a room overflows?

What we'll learn

• Add two three-digit numbers with regrouping.
• Understand why we "carry over".
• Place numbers on a number line.
• Round to the nearest ten or hundred.
• Reach 1000 and meet four-digit numbers briefly.

The big idea

Each room in the House of Hundreds can hold only 9 of its kind. The ones room can hold 9 ones , but if you try to put a 10th, those 10 ones bundle into a new ten and move up. That is regrouping.

When we add 156 + 137, the ones room gets 6 + 7 = 13. That is one full ten and 3 ones. We write 3 in the ones room and move the 1 ten up. Then 5 + 3 + 1 = 9 tens. Hundreds: 1 + 1 = 2. The answer is 293.

What happens when even the hundreds room overflows? 950 + 100 = 1050. The hundreds room cannot hold 10 hundreds. They bundle into a new room , the thousands. Welcome to four-digit numbers!

The number line is a useful picture. Numbers grow as we move right. 250 is to the right of 240 and to the left of 260. Rounding to the nearest ten means finding the closest ten on the line.

Let's explore

Regroup with sticks. Take bundles of ten matchsticks. Put 8 sticks in the ones pile. Add 5 more. You will see , 10 of them tie up into a new bundle of ten.

Walk to 1000. Start at 990 and count up: 991, 992, 993... 998, 999, 1000. What number comes right after 999?

Number line jumps. Draw a line from 0 to 200 in jumps of 10. Mark 70, 130 and 180. Which is closest to 100?

Add the bus tickets. Imagine 2 bus tickets , one costs 245 rupees, the other 178. What is the total?

Worked examples

Example 1. Add 248 + 376.

• Ones: 8 + 6 = 14. Write 4, carry 1 ten.
• Tens: 4 + 7 + 1 = 12. Write 2, carry 1 hundred.
• Hundreds: 2 + 3 + 1 = 6.
• Answer: 624.

Example 2. Round 87 to the nearest ten.

• 87 is between 80 and 90.
• 87 is closer to 90 (only 3 away from 90, but 7 away from 80).
• Rounded: 90.

Example 3. What number is one more than 999?

• 999 has 9 hundreds, 9 tens, 9 ones.
• Add 1 → 9 ones become 10 ones → 1 ten, 0 ones.
• 9 tens + 1 ten = 10 tens → 1 hundred, 0 tens.
• 9 hundreds + 1 hundred = 10 hundreds → 1 thousand.
• Answer: 1000.

Quick check

1. Add: 359 + 246 = ?
2. What comes after 599?
3. Round 64 to the nearest ten.
4. Is 478 closer to 400 or 500?
5. Write the number that is 10 less than 1000.

Recap

• When a room of the House of Hundreds is full, the next ten bundles up and moves to the higher room.
• This is called carrying or regrouping.
• After 999 comes 1000 , and a brand new room opens.