SME
Grade 3 Math · Problem type
Estimate sums
Use this for “About how many?” and to check a sum by rounding.
Diagram
238 rounds to 240.
154 rounds to 150.
Estimate: 240 + 150 = 390.
First rounded: 240
Second rounded: 150
Estimate: 390
How far off the estimate is: 2
This is an example. Type your own values to start a new problem.
First number
example
Second number
example
First rounded
worked out (not typed)
Second rounded
worked out (not typed)
Estimate
worked out (not typed)
Sum
worked out (not typed)
How far off the estimate is
worked out (not typed)
Formulas
238 rounds to 240
First number rounds to first rounded
154 rounds to 150
Second number rounds to second rounded
240 + 150 = 390
First rounded + second rounded = estimate
238 + 154 = 392
First number + second number = sum
The estimate 390 is 2 away from the sum 392
Sum − estimate = how far off the estimate is (or the other way round)
Assumptions
•
Round each number to the nearest ten, then add the rounded numbers.
•
The estimate is close to the real sum. Use it to check the answer makes sense.
Step-by-step
WE KNOW
First number: 238
Second number: 154
FIND
first rounded, second rounded, estimate, sum, how far off the estimate is
Step 1 · Find first rounded
238 rounds to ?
First number rounds to first rounded
Round the first number to the nearer ten. 5 ones or more rounds up.
238 is between 230 and 240.
8 ones is 5 or more: round up to 240.
First rounded = 240
Step 2 · Find second rounded
154 rounds to ?
Second number rounds to second rounded
Round the second number to the nearer ten. 5 ones or more rounds up.
154 is between 150 and 160.
4 ones is less than 5: round down to 150.
Second rounded = 150
Step 3 · Find estimate
240 + 150 = ?
First rounded + second rounded = estimate
Add the first rounded and the second rounded.
2
4
0
+
1
5
0
3
9
0
Estimate = 390
Step 4 · Find sum
238 + 154 = ?
First number + second number = sum
Add the first number and the second number.
1
2
3
8
+
1
5
4
3
9
2
Ones: 8 + 4 = 12
Tens: 30 + 50 = 80
Hundreds: 200 + 100 = 300
Sum = 392
Step 5 · Find how far off the estimate is
The estimate 390 is ? away from the sum 392
Sum − estimate = how far off the estimate is (or the other way round)
Take the smaller from the bigger. A small distance means the estimate is close.
How far off the estimate is = 392 − 390
Exact: 238 + 154 = 392
392 − 390 = 2
How far off the estimate is = 2
Check
Put the numbers back in. Both sides should match.
238 rounds to 240 ✓
154 rounds to 150 ✓
240 + 150 = 390 ✓
238 + 154 = 392 ✓
The estimate 390 is 2 away from the sum 392 ✓
Topic check
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