Example:

363627
- 54564
	Estimate

Step 1:   Identify which place value you need to round to.
Round to the largest place value of the number with the fewest digits.

363627 has 6 digits and 54564 has 5 digits.
54564 has fewer digits and its largest place value is the ten-thousands.

Step 2:   Round the numbers to the correct place value found in Step 1.
We're rounding to the nearest ten-thousands.

363627 rounds to 360000.
  54564 rounds to   50000.

Step 3:   Subtract the rounded numbers to get the estimated difference.

360000

+ 50000

310000

The answer is

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