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E.1 Estimate sums by rounding to different place values

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What does it mean to estimate sums by rounding?

Estimating sums by rounding means finding an approximate total by rounding numbers to a specific place value before adding. This makes addition easier and quicker without needing the exact answer.

Example:
  • 326 + 487 → round both to the nearest hundred → 300 + 500 = 800
Note

Estimation helps check if an exact answer makes sense or when only an approximate total is needed.

How do you identify where to round?

To identify where to round, look at the place value you are asked to round to (tens, hundreds, thousands, ten thousands, or hundred thousands). Then check the digit to the right to decide whether to round up or keep the number the same.

Steps:
  • Round to the nearest 10: look at the ones place.
  • Round to the nearest 100: look at the tens place.
  • Round to the nearest 1,000: look at the hundreds place.
  • Round to the nearest 10,000: look at the thousands place.
  • Round to the nearest 100,000: look at the ten-thousands place.
Note

If the digit to the right is 5 or greater, round up. If it is 4 or less, keep the digit the same.

Examples of estimating sums

Here are examples of estimating sums by rounding to different place values.

Examples:
  • 452 + 379 → round to nearest 10 → 450 + 380 = 830
  • 452 + 379 → round to nearest 100 → 500 + 400 = 900
  • 7,236 + 15,482 → round to nearest 1,000 → 7,000 + 15,000 = 22,000
  • 47,529 + 62,481 → round to nearest 10,000 → 50,000 + 60,000 = 110,000
  • 234,678 + 321,456 → round to nearest 100,000 → 200,000 + 300,000 = 500,000
Note

Estimation will not give the exact answer, but it shows about how much the sum will be.

When should you use estimation?

Use estimation when you want a quick answer, to check if your exact calculation is reasonable, or when the exact number is not needed.

Examples of when to estimate:
  • Checking if a total makes sense after solving a problem.
  • Adding prices while shopping to see about how much you will spend.
  • Finding about how many people are at an event.
Note

Estimation is a helpful skill in real life when exact answers are not necessary.

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