Mentally adding tens to 3-digit numbers · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Adding and subtracting tens in your head

Mathematics · Addition & Subtraction · ages 7-8
Name ______________________   Date ____________

  1. What is 462 − 30?

    Answer: ______________

  2. What is 345 + 40?

    • 349
    • 745
    • 375
    • 385

  3. What is 354 + 40?

    350360370380390400+40
    • 358
    • 394
    • 754

  4. When you add tens to a 3-digit number, the ones digit changes.

    Circle one:   True   False

  5. What is 530 − 40?

    Answer: ______________

  6. Calculate 745 − 20.

    Answer: ______________

  7. What is 278 + 50?

    • 328
    • 323
    • 728
    • 283

  8. Jack says 278 + 50 = 273. Is Jack right?

    Circle one:   True   False

  9. Zoe says 480 + 30 = 780. What mistake did she make?

    470480490500510520+30
    • She added 30 as if it were 300
    • She forgot to add the ones digit
    • She made no mistake

  10. What is 460 + 90?

    Answer: ______________

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Adding and subtracting tens in your head W1-mt_aHQ9kNt3is-s1

  1. 432 · Taking 30 shrinks the 6 tens to 3 tens, giving 432.
  2. 385 · Adding 40 grows the 4 tens to 8 tens, giving 385.
  3. 394 · The tens digit moves from 5 to 9. Everything else stays the same: 354 + 40 = 394.
  4. False · Adding tens changes the tens digit, and sometimes the hundreds digit. The ones digit stays the same.
  5. 490 · Taking 40 shrinks the 3 tens to zero tens with a hundred break: 530 − 40 = 490.
  6. 725 · The tens digit drops from 4 to 2: 745 − 20 = 725.
  7. 328 · Adding 50 grows the 7 tens past a hundred boundary: 278 + 50 = 328.
  8. False · Jack is wrong. Adding 50 grows the total to 328, not 273.
  9. She added 30 as if it were 300 · 480 + 30 is 510: 8 tens plus 3 tens makes 11 tens, which rolls 1 hundred and leaves 1 ten. Zoe's answer of 780 treated the 3 as hundreds instead of tens.
  10. 550 · Adding 90 pushes the 6 tens over the hundred bridge: 460 + 90 = 550.
Worksheet · LightMySky