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Step 5: Multiplication and first fractions

Rounding, and adding and subtracting within 1,000

Step 5 · Multiplication and first fractions topics

Round numbers to the nearest 10 or 100 on a number line, and add and subtract within 1,000 place by place, with an estimate to check.

Rounding to the nearest ten and hundred

Rounding swaps a number for a nearby one that is easier to work with. To round 247 to the nearest ten, ask: is it nearer 240 or 250?

247 is 7 away from 240 and 3 away from 250, so it rounds to 250.

Look at the digit just after the place you are rounding to. If it is 5 or more, round up; if it is 4 or less, round down. A number exactly halfway, like 245, rounds up to 250.

Example: Rounding to the nearest hundred

Round 683 to the nearest hundred.

  1. 683 is between 600 and 700.
  2. The tens digit is 8, which is 5 or more.
  3. So round up.

Answer700

Adding within 1,000

Add hundreds to hundreds, tens to tens and ones to ones. When a place makes 10 or more, regroup: 10 ones make a ten, and 10 tens make a hundred.

358 and 276 as hundreds, tens and ones. The 14 ones make a new ten; the 13 tens make a new hundred.

Example: Adding with regrouping

What is 358 + 276?

  1. Ones: 8 + 6 = 14. Write 4 and carry 1 ten.
  2. Tens: 5 + 7 + 1 = 13. Write 3 and carry 1 hundred.
  3. Hundreds: 3 + 2 + 1 = 6.

Answer634

Check with an estimate: rounded to tens, 360 + 280 = 640, which is close to 634.

Subtracting within 1,000

Subtract place by place, starting with the ones. If the top digit is too small, regroup: take 1 from the next place to the left.

Example: Subtracting with regrouping

What is 523 − 167?

  1. Ones: 3 − 7 cannot be done. Take a ten: 13 − 7 = 6.
  2. Tens: 1 − 6 cannot be done. Take a hundred: 11 − 6 = 5.
  3. Hundreds: 4 − 1 = 3.

Answer356

Or count up from the smaller number: from 167 to 200 is 33, and from 200 to 523 is 323. 33 + 323 = 356.

Try it yourself

Work each one on paper first, then open the answer to check.

  1. Round 74 to the nearest ten.

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    70.

  2. Round 450 to the nearest hundred.

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    500. It is exactly halfway, so it rounds up.

  3. What is 247 + 385?

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    632.

  4. What is 600 − 254?

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    346.

  5. Estimate 512 − 289 by rounding to the nearest hundred.

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    500 − 300 = 200.

What next?