SME
Grade 5 Math · Problem type
Estimate products of decimals
Use this for “About how much is 4.8 × 6.2?” by rounding each to the nearest whole number.
Diagram
5 × 6 = 30.
This is an example. Type your own values to start a new problem.
First number
×
Second number
≈
First number rounded
×
Second number rounded
=
Estimated product
Formulas
4.8 rounded to the ones: 5
The first number rounded to the nearest whole = first number rounded
6.2 rounded to the ones: 6
The second number rounded to the nearest whole = second number rounded
5 × 6 = 30
First number rounded × second number rounded = estimated product
Assumptions
•
Round each decimal to the nearest whole number: 5 tenths or more rounds up.
•
Multiply the rounded numbers. The estimate is close to the exact product.
•
Use the estimate to check that an exact answer makes sense. Numbers from 1 to 999.99.
Step-by-step
WE KNOW
First number: 4.8
Second number: 6.2
FIND
first number rounded, second number rounded, estimated product
Step 1 · Find first number rounded
4.8 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.
4.8 is between 4 and 5 → 5
First number rounded = 5
Step 2 · Find second number rounded
6.2 rounded to the ones: ?
The second number rounded to the nearest whole = second number rounded
Round the second number the same way.
6.2 is between 6 and 7 → 6
Second number rounded = 6
Step 3 · Find estimated product
5 × 6 = ?
First number rounded × second number rounded = estimated product
Multiply the whole numbers in your head.
Estimated product = 30 (the exact product is 29.76)
Check
Put the numbers back in. Both sides should match.
4.8 rounded to the ones: 5 ✓
6.2 rounded to the ones: 6 ✓
5 × 6 = 30 ✓
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Estimate differences of decimals
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Add and subtract fractions with unlike denominators
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Related lessons
Add, subtract, multiply and divide decimals
Main lesson
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