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10 minutes, then stop

Maths B (new unit)Fri 2 October1 / 4

Rounding

About how many? — worked out by a fixed rule

Name Date

The rounding rule

To round to the nearest hundred, look only at the next place down (the tens).
0, 1, 2, 3, 4 → round down (keep it, zeros below)
5, 6, 7, 8, 9 → round up (add 1 to the hundreds, zeros below)
"Round the ones digit" means the same as "round to the nearest ten".

Question 1 is already done for you

Round 2,738 to the nearest hundred.

① The hundreds digit is 7. One place down, the tens digit is 3
② 3 is 4 or less → round down
③ Answer 2,700

Your turn from here

2Round 5,162 to the nearest hundred.

3Round 8,349 to the nearest hundred.

Do not look at the 9 in the ones.

4Round 38,490 to the nearest thousand.

To the nearest thousand means looking at the hundreds — not at the 9.

10 minutes, then stop

Maths B (new unit)Fri 2 October2 / 4

Rounding

Name Date

A common mistake

Round 3,450 to the nearest hundred.
Looking at the 0 in the ones: 3,400　→ wrong place. The tens digit is 5 → up → 3,500
Round 3,450 to the nearest thousand.
3,450 → 3,500 → 4,000　→ never round twice. The hundreds digit is 4 → 3,000

A number exactly in the middle is always rounded up.

5Round 6,950 to the nearest hundred.

What happens when you add 1 to the 9 in the hundreds?

6Round 24,499 to the nearest thousand.

Rounding to the first few digits

First 2 digits → round with the 3rd digit from the left (first 1 digit → the 2nd).
Example　6,380 → the 3rd digit is 8 → up → 6,400

7Round 47,815, keeping the first 2 digits.

10 minutes, then stop

Maths B (new unit)Fri 2 October3 / 4

Rounding

Name Date

"At least" and "less than"

5 or more (at least 5) … includes 5 (5, 6, 7 …)
less than 5 … does not include 5 (4, 3, 2 …)
5 or less (at most 5) … includes 5 (5, 4, 3 …)

8Rounded to the nearest ten, a whole number becomes 80. What are the smallest and the largest it could be?

Draw a number line and look either side of 80.

9Numbers that round to 600 (nearest hundred): write their range using "at least" and "less than".

10Which of these round to 3,000 (nearest hundred)? Choose all of them.
　A 2,950　B 3,049　C 2,949　D 3,050

10 minutes, then stop

Maths B (new unit)Fri 2 October4 / 4

Rounding

Name Date

Estimating — a rough answer first

11You buy three things: 198 yen, 312 yen and 489 yen. Round each to the nearest hundred and estimate the total.

12You buy things for 289 yen, 305 yen and 196 yen. To know whether 1,000 yen is enough, round each up to the hundred (whatever the tens digit, add 1 to the hundreds) and estimate. Is it enough?

Rounding up makes it bigger than the real total. If that is still enough, you are safe.

13You buy 21 sweets at 48 yen each. Round both numbers, keeping only the first digit, and estimate the cost.