Week 2 · Comparing, ordering and rounding big numbers · 3 lessons
3
Rounding rules for big numbers
Discover Mathematics P4: Term 1, Unit 1, Lessons 7–11
GoalRound big numbers to the nearest ten, hundred, thousand, ten thousand or million, and use rounding to estimate.
New words
round · to change a number to a nearby number that is easier to useestimate · an answer that is close to the exact answerrounding place · the place we round to, for example the hundreds place
Explanation
Rounding gives a number that is easier to say and use. Step 1: find the rounding place and underline its digit. Step 2: look at the digit just to its right. Step 3: if that digit is 0, 1, 2, 3 or 4, keep the underlined digit the same. If it is 5, 6, 7, 8 or 9, make the underlined digit one bigger. Step 4: change every digit to the right of the rounding place to 0. A number exactly half way, like 450, goes up to 500. On a number line, the rounded number is the nearer ten, hundred or thousand. To estimate an answer, round the numbers first and then add or subtract the easy numbers.
Worked examples
Round 6,482 to the nearest hundred. The hundreds digit is 4 and the digit to its right is 8. Since 8 is 5 or more, the 4 becomes 5 and the rest become zeros: 6,500. On the number line 6,482 is closer to 6,500 than to 6,400.
Number
Round to
Result
38,260
thousand
38,000
2,950,000
million
3,000,000
Round 38,260 to the nearest thousand. The thousands digit is 8 and the digit to its right is 2. Since 2 is less than 5, the 8 stays: 38,000. Round 2,950,000 to the nearest million. The millions digit is 2 and the next digit is 9, so 2 becomes 3: 3,000,000.
Number
Rounded
48,720
49,000
31,180
31,000
Sum
80,000
Estimate 48,720 + 31,180. Round each number to the nearest thousand: 49,000 and 31,000. Then 49,000 + 31,000 = 80,000. The exact answer is 79,900, so the estimate is very close.
Class activity
Rounding shop. Look at three prices or amounts at home, for example the distance in km between two cities or the number of pages in a big book. Round each to the nearest ten and to the nearest hundred. Which rounded number tells the story better? Why?
Practice
1
Round 7,364 to the nearest ten, hundred and thousand.
7,360, 7,400 and 7,000.
2
Round 85,500 to the nearest thousand.
86,000, because the digit to the right of the thousands is 5.
3
Round 4,999,999 to the nearest million.
5,000,000.
4
A number rounded to the nearest ten is 70. What is the smallest whole number it could be?
65, because 65 rounds up to 70 and 64 rounds down to 60.
5
Marwan rounds 2,449 to the nearest hundred and gets 2,500. What did he do wrong?
He looked at more than one digit. Only the digit to the right of the hundreds place counts: 4, so the answer is 2,400.
