Developing Mental Math Strategies: Rounding 2-Digit Numbers to the Nearest Ten
A Grade 3 SMART Math Sequence by Christine Blessin (SD#33 Chilliwack)
GOAL: Round numbers to the nearest ten to develop mental math strategies for
addition and subtraction of 2-digit numbers.
BC IRP PLOs:
A6 Describe and apply mental mathematics strategies for adding two 2-digit numerals
such as taking one addend to the nearest multiple of ten and then compensating
A7 Describe and apply mental mathematics strategies for subtracting two 2-digit
numerals such as taking one subtrahend to the nearest multiple of ten and then
compensating
CONNECT:
APK: To be able to round numbers to the nearest ten, you have to know how to count by
tens. Put your hand up if you feel like you’re an expert at that by now. Choose 11
students with hands raised to come up to the front (for a surprise demo).
Students who have “volunteered” will become a human number line. Hand out small white
boards/felts to each and have them each write a numeral 0-100 (counting by tens) on the
whiteboards. Line the students up with a fair sized gap between each child.
0
10
20
30
40
50
60
70
80
90
100
Distribute numeral cards to 4 of the other seated students. Cards should read 8, 17, 62 and 95.
Ask: “Who has a numeral that comes between 0 and 10?”
“Can you come and high-five the person on the number line who you are nearer to?
0 or 10? Tell me how you know you’re nearer to 10.”
“Great! You just rounded the number 8 to the nearest ten!”
Repeat for the numbers 17 and 62.
Have students place white boards on the floor and return to their desks. Have the student who
has 95 hold up the number and ask, “What about this number?”
ESTIMATE (PREDICT): On the ANIE© assessment sheet, have students estimate whether 95
will round up to 100 or down.
For the student who has the number 95, repeat the same process but when asking “How do you
know you’re closer to 100 than 90” wait for some students to say “Well, that’s just the rule” and
then ask…
QUESTION: “How do you think mathematicians came up with that rule? What do you
wonder about that rule?
Tell students “In math, it’s important to remember certain rules and I’ll draw you a picture to
help you remember why an in between number like 95, that ends with a 5 always rounds UP.
When I checked to see how much you knew about rounding, most of you drew pictures of
number lines and wrote about numbers being closer to the lower ten or higher ten. But what do
you do when you have a number like 95 that’s right in the middle? My picture will help you
remember…
PROCESS:
1. Show students the “car rolling back down/ over the hill” idea on the whiteboard. If you
get out of the car at the top of the hill (number that ends in 5) to look at the view and you
forget to put the car in park, which way will it roll? The engine would be heavier than
the trunk, so the car would roll forward = the number would round to the higher ten.
Demonstrate rounding the number 33 with small magnetic car on white board.
(Chunk #1)
Have students illustrate this example in the drawing section of the ANIE© assessment
sheet. Change the numbers on the hill (90-100) for the purpose of students’ illustrations.
Encourage students to use arrows to show the direction the car would roll if it stopped at
the number 95.
2. There’s also great song to help you remember how to round to the nearest ten.
Army chant style:
Rounding numbers is so fun, (repeat)
We will do it ‘till we’re done. (repeat)
If a number’s 0-4, (repeat)
Round it down to the ten before (repeat)
If a number’s 5-9, (repeat)
Round it UP the number line. (repeat)
Round UP (repeat)
Round DOWN (repeat)
1, 2, 3, 4
1, 2…3, 4!
(Chunk #2)
Have students write what is important to remember from this song in the writing section of
the ANIE© assessment sheet (basically the rule for rounding to the nearest ten).
3. You can think of numbers being with and without power
Numbers 0-4 don’t have power but 5-9 have power.
Let’s look at the number 23. When you round to the nearest ten, you look at the number
in the ones place and ask yourself “Does 3 have power?” If it doesn’t, then it “poofs
away” and becomes a zero. 23 rounds down to 20. The 3 doesn’t have the power to
change the number in the tens place.
Let’s look at the number 58. Ask yourself “Does 8 have power?” If it does, (draw a
lightning bolt under the number) it magically has the power to change the number in the
tens place. 58 becomes the higher ten: 60!
Partner Activity:
In pairs, take turns writing a two digit number on your white board for your partner to round to
the nearest ten. Remember to draw a lightning bolt under numbers in the ones place that “have
power”. This will change the number in the tens place.
Re-visit the human number line now and have the same students write new numbers on their
white boards.
100
110
120
130
140
150
160
170
180
190
200
Distribute the numbers 106, 119, 172, 145 to seated students. Repeat “high-five” process, same
as before. Ask the class “What do you notice about rounding numbers, even though our number
line has changed?”
TRANSFORM -- Part One
Now we need to answer the following question: “Why do we need to be able to round numbers
to the nearest ten in real life? Why are we learning this anyway?”
Rounding numbers gives us approximate amounts.
• 22 cents in my pocket would be about 20 cents (I would round down)
• If it takes 17 minutes to walk to school, I would round up and say it takes about 20
minutes
• If the distance to Abbotsford is 32 kilometers, I would round down and say it’s about 30
• If there were 45 kids playing on the playground, I would round up and say that about 50
kids were there
Rounding numbers or amounts of things also makes it easier for our brains to do mental math,
especially when we’re adding and subtracting.
Example:
There are 47 pink jelly beans and 95 lollipops in a jar. About how many candies are there in
total? Stress that the answer is approximate (close to the real answer).
Even if you only round one of the numbers and then add them together, it will be much easier
and faster than adding with regrouping. Math is the pursuit of laziness!! 
Choose the greater number to round (95 becomes 100) and then add the lesser number (47).
100+47=147. There are about 147 candies in the jar.
Do another example with the class. This time, do a subtraction question and have students come
up with the subtrahends and what they represent (they create the mental math problem). Solve as
a class.
Have students get into partners to make their own “real life” adding question (2-digit + 2-digit
and round the higher number) to get an approximate answer. Students don’t have to worry about
compensating yet. Have students describe their equation in writing in the real life application
section of the ANIE©. Invite some students to share their examples at the end.
REFLECT: Did we achieve our goal?
Do you have any new ideas about 2-digit numbers after today’s lesson?
What was easy? What was hard?
Goal for next time?
(Have students discuss new ideas and reflect on their thinking with an AB partner before writing
in the reflection portion of the ANIE©)
TRANSFORM – Part Two
Students complete practice pages in class. This includes “real-life” adding/subtracting
problems where students will have to round one number to facilitate adding/subtracting mentally.
ASSESSMENT:
For learning:
Prior to this lesson, students completed the ANIE© assessment with the problem “Round 57 to
the nearest ten”. Students’ comprehension (in concepts and applications, strategies and
approaches, accuracy and representation) of rounding was marked using the Performance
Standards rating scale of 1, 2, 3 and 4.
Some findings:
Most students struggled to give an example of how rounding could be used in “real life”.
Most students used money in their attempted real life examples.
Most students related rounding to a number line.
As learning:
The ANIE© assessment sheet is used throughout the lesson as a tool that captures student
comprehension of the lesson as it is being delivered. Students work in pairs to complete the most
challenging task (the real life math application).
Of learning:
After necessary practice, students will be given the ANIE© assessment once again with a new
problem: “Round 45 to the nearest ten”. Students will complete the entire assessment
individually. Student comprehension will again be graded using the Performance Standards
rating scale.
A student who is fully meeting expectations would draw the visual of the car on the hill to
explain why 5 rounds UP to the higher ten. The student would also be able to give an example
of adding/subtracting 2-digit numerals mentally by rounding one addend/subtrahend and then
compensating (the example could be one we discussed in class) Students should be able to
explain that rounding numbers makes them easier to add/subtract.
A student who exceeds expectations would give a new real life math example (not previously
discussed in class).