SME
Grade 4 Math · Problem type
Estimate sums
Use this for “About how many in all? 36,325 + 23,310” by rounding first.
Diagram
36,000 + 23,000 = 59,000
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 sum
worked out (not typed)
Formulas
36,325 rounded to the 1,000s: 36,000
The first number rounded to the place = first number rounded
23,310 rounded to the 1,000s: 23,000
The second number rounded to the place = second number rounded
36,000 + 23,000 = 59,000
First number rounded + second number rounded = estimated sum
Assumptions
•
Round each number to the same place: hundreds, thousands or ten thousands.
•
Add the rounded numbers. The estimate is close to the real sum.
•
Use the estimate to check an exact answer: they should be close.
Step-by-step
WE KNOW
First number: 36,325
Second number: 23,310
Round to: 1,000
FIND
first number rounded, second number rounded, estimated sum
Step 1 · Find first number rounded
36,325 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.
36,325 is between 36,000 and 37,000
36,325: the hundreds digit is 3, less than 5 → 36,000
First number rounded = 36,000
Step 2 · Find second number rounded
23,310 rounded to the 1,000s: ?
The second number rounded to the place = second number rounded
Round the second number to the same place.
23,310 is between 23,000 and 24,000
23,310: the hundreds digit is 3, less than 5 → 23,000
Second number rounded = 23,000
Step 3 · Find estimated sum
36,000 + 23,000 = ?
First number rounded + second number rounded = estimated sum
Add the rounded numbers. Rounded numbers are easy to add in your head.
Estimated sum = 59,000 (the exact sum is 59,635)
Check
Put the numbers back in. Both sides should match.
36,325 rounded to the thousands: 36,000 ✓
23,310 rounded to the thousands: 23,000 ✓
36,000 + 23,000 = 59,000 ✓
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Place value and rounding to one million
Main lesson
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Use this for sums to 1,000,000 in columns, like 36,325 + 23,310.
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Subtract across zeros
Use this for “Subtract: 5,000 − 2,346” when the start has zeros to regroup across.