Rounding decimal numbers
🎯 In this topic you will
- Round numbers with 1 decimal place to the nearest whole number
🧠 Key Words
- nearest
- round
- round to the nearest …
Show Definitions
- nearest: The value that is closest to a given number when comparing two possible rounding options.
- round: To change a number so that it becomes simpler but still close to its original value.
- round to the nearest …: A rounding method where a number is adjusted to the closest whole number, tenth, hundredth, and so on depending on what place value is being used.
Why We Round Numbers
Rounding makes it easier to describe and understand numbers. It is easier to understand ‘an African elephant is approximately 3 metres tall’ than ‘the average height of an African elephant is 3.3 metres’.
🔍 Can you round these heights to the nearest whole number?
- Height of elephant = 3.3 m
- Height of giraffe = 5.5 m
- Height of ostrich = 2.7 m
Answer 1
3.3 m rounds to 3 m.
Answer 2
5.5 m rounds to 6 m.
Answer 3
2.7 m rounds to 3 m.
❓ EXERCISES
1. Identify the numbers marked by arrows. Round each number to the nearest whole number.
👀 Show answer
A: The arrow is at $2.6$, which rounds to $3$.
B: The arrow is at $5.5$, which rounds to $6$.
C: The arrow is at $8.1$, which rounds to $8$.
2. Round these numbers to the nearest whole number.
a. $65.8$
b. $101.1$
c. $44.4$
d. $55.5$
👀 Show answer
a. $65.8 \rightarrow 66$
b. $101.1 \rightarrow 101$
c. $44.4 \rightarrow 44$
d. $55.5 \rightarrow 56$
3. Round these measures to the nearest whole centimetre.
a. $2.8\ \text{cm}$
b. $8.5\ \text{cm}$
Round these measures to the nearest whole metre.
c. $7.3\ \text{m}$
d. $0.3\ \text{m}$
👀 Show answer
a. $2.8\ \text{cm} \rightarrow 3\ \text{cm}$
b. $8.5\ \text{cm} \rightarrow 9\ \text{cm}$
c. $7.3\ \text{m} \rightarrow 7\ \text{m}$
d. $0.3\ \text{m} \rightarrow 0\ \text{m}$
4. A number with $1$ decimal place is rounded to the nearest whole number.
a. What is the smallest number that rounds to $5$?
b. What is the largest number that rounds to $5$?
Ask your partner to check your answers. Did you both choose the same numbers?
👀 Show answer
a. The smallest number (with $1$ decimal place) that rounds to $5$ is $4.5$.
b. The largest number (with $1$ decimal place) that rounds to $5$ is $5.4$.
5. $74.5$ is $74$ to the nearest whole number.
Is Marcus correct? Explain your answer.
👀 Show answer
Marcus is not correct. $74.5$ rounds to $75$, not $74$, because a tenths digit of $5$ means you round up to the next whole number.
6. Use a calculator to help you answer this question.
Two numbers each with $1$ decimal place round to $231$ to the nearest whole number.
The sum of the two numbers is $462$.
What could the numbers be?
👀 Show answer
One possible pair is $230.9$ and $231.1$.
Each has $1$ decimal place and rounds to $231$, and $230.9 + 231.1 = 462$.
7. Use the clues to identify the correct number.
The possible numbers are $10.42$, $9.73$, $9.9$, $9.37$, $7.83$, $10.1$, $8.7$, $9.55$.
The number:
- has a tenths digit which is odd,
- rounds to $10$ to the nearest whole number,
- has no hundredths digit,
- is less than $10$.
Swap books with your partner and check their answers.
👀 Show answer
The correct number is $9.9$.
It has an odd tenths digit ($9$), it rounds to $10$ to the nearest whole number, it has no hundredths digit, and it is less than $10$.
🧠 Think like a Mathematician
Task: Roll a dice twice and use the two digits to make a number with $1$ decimal place.
Method:
- Roll a dice twice and form a number with $1$ decimal place (e.g., a $2$ followed by a $5$ becomes $2.5$).
- List all the different numbers you can make.
- Round each number to the nearest whole number.
- Roll the dice again and make more numbers.
- Record which whole numbers the rounded results match.
Follow-up Questions:
Show Answers
- 1: All possible one-decimal-place numbers from dice rolls are $1.1$ to $6.6$, including combinations like $2.5$, $5.2$, $3.4$, etc.
- 2: These numbers can round to $1$, $2$, $3$, $4$, $5$, or $6$ depending on the tenths digit.
- 3: If the tenths digit is $0$–$4$, the number rounds down; if the tenths digit is $5$–$9$, the number rounds up.