SME
Grade 5 Math · Problem type
Estimate sums of decimals
Use this for “About how much is 12.65 + 8.3?” by rounding each to the nearest whole number.
Diagram
13 + 8 = 21
This is an example. Type your own values to start a new problem.
First number
+
Second number
≈
First number rounded
+
Second number rounded
=
Estimated sum
Formulas
12.65 rounded to the ones: 13
The first number rounded to the nearest whole = first number rounded
8.3 rounded to the ones: 8
The second number rounded to the nearest whole = second number rounded
13 + 8 = 21
First number rounded + second number rounded = estimated sum
Assumptions
•
Round each decimal to the nearest whole number: 5 tenths or more rounds up.
•
Add the rounded numbers. The estimate is close to the exact sum.
•
Use the estimate to check that an exact answer makes sense. Numbers from 1 to 999.99.
Step-by-step
WE KNOW
First number: 12.65
Second number: 8.3
FIND
first number rounded, second number rounded, estimated sum
Step 1 · Find first number rounded
12.65 rounded to the ones: ?
The first number rounded to the nearest whole = first number rounded
Look at the tenths digit. 5 or more rounds up to the next whole number.
12.65 is between 12 and 13 → 13
First number rounded = 13
Step 2 · Find second number rounded
8.3 rounded to the ones: ?
The second number rounded to the nearest whole = second number rounded
Round the second number the same way.
8.3 is between 8 and 9 → 8
Second number rounded = 8
Step 3 · Find estimated sum
13 + 8 = ?
First number rounded + second number rounded = estimated sum
Add the whole numbers in your head.
Estimated sum = 21 (the exact sum is 20.95)
Check
Put the numbers back in. Both sides should match.
12.65 rounded to the ones: 13 ✓
8.3 rounded to the ones: 8 ✓
13 + 8 = 21 ✓
Topic check
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Related lessons
Add, subtract, multiply and divide decimals
Main lesson
Multiply decimals and whole numbers
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Divide decimals by whole numbers
Use this for 7.2 ÷ 4: a decimal shared into equal parts.
Subtract decimals
Use this for “Subtract: 5.27 − 2.85 = ___.”
Multiply a decimal by a decimal
Use this for “What is 0.6 × 0.3?” with tenths times tenths.
Divide by a decimal
Use this for “How many 0.5s are in 3?” (3 ÷ 0.5), or 0.24 ÷ 0.08.
Estimate differences of decimals
Use this for “About how much is 12.65 − 8.3?” by rounding each to the nearest whole number.
Estimate products of decimals
Use this for “About how much is 4.8 × 6.2?” by rounding each to the nearest whole number.