SME
Grade 4 Math · Problem type
Estimate differences
Use this for “About how many more? 61,209 − 27,845” by rounding first.
Diagram
33,000 + 28,000 = 61,000
This is an example. Type your own values to start a new problem.
First number
example
Second number
example
Round to
example
First number rounded
worked out (not typed)
Second number rounded
worked out (not typed)
Estimated difference
worked out (not typed)
Formulas
61,209 rounded to the 1,000s: 61,000
The first number rounded to the place = first number rounded
27,845 rounded to the 1,000s: 28,000
The second number rounded to the place = second number rounded
61,000 − 28,000 = 33,000
First number rounded − second number rounded = estimated difference
Assumptions
•
Round each number to the same place: hundreds, thousands or ten thousands.
•
Subtract the rounded numbers. The estimate is close to the real difference.
•
Use the estimate to check an exact answer: they should be close.
Step-by-step
WE KNOW
First number: 61,209
Second number: 27,845
Round to: 1,000
FIND
first number rounded, second number rounded, estimated difference
Step 1 · Find first number rounded
61,209 rounded to the 1,000s: ?
The first number rounded to the place = first number rounded
Look at the digit to the right of the place. 5 or more rounds up.
61,209 is between 61,000 and 62,000
61,209: the hundreds digit is 2, less than 5 → 61,000
First number rounded = 61,000
Step 2 · Find second number rounded
27,845 rounded to the 1,000s: ?
The second number rounded to the place = second number rounded
Round the second number to the same place.
27,845 is between 27,000 and 28,000
27,845: the hundreds digit is 8, 5 or more → 28,000
Second number rounded = 28,000
Step 3 · Find estimated difference
61,000 − 28,000 = ?
First number rounded − second number rounded = estimated difference
Subtract the rounded numbers. Rounded numbers are easy to subtract in your head.
Estimated difference = 33,000 (the exact difference is 33,364)
Check
Put the numbers back in. Both sides should match.
61,209 rounded to the thousands: 61,000 ✓
27,845 rounded to the thousands: 28,000 ✓
61,000 − 28,000 = 33,000 ✓
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