Use this page when you want number bonds to 20 worksheets in the next thirty seconds: choose how many problems you want, press print, and the answer key comes out on its own page behind the worksheet. The current sheet starts with problems like 4 + □ = 20 and 5 + □ = 20, and the answers (16 and 15) are printed on the key so marking takes about a minute.
‘New set of problems’ opens the generator in its own tab. There you can change the problem count and turn the answer key on or off. ‘Print or save as PDF’ opens the print dialog — choose Save as PDF in the printer list to keep the file. The sheet fits US Letter and A4, and the answer key prints on its own page.
The sheet mixes the position of the missing value, so the box sometimes sits in the first slot and sometimes in the answer slot. Children who have only met one arrangement usually stall on the other.
Missing-number work also exposes memorised facts that are not really known. A child can recite a fact forwards and still be unable to produce 15 when the same fact is presented backwards.
The sheet uses pairs that make twenty, so the difficulty is the reasoning, not the arithmetic.
A missing-number problem such as 4 + □ = 20 asks the child to work backwards, and that is a genuinely different skill from computing forwards. The answer 16 cannot be reached by reading left to right and doing what the sign says.
In US classrooms these number bonds to 20 worksheets are commonly aligned with the operations and algebraic thinking strand for this grade, though the exact wording varies from state to state.
| # | Problem | Answer | What to check |
|---|---|---|---|
| 1 | 4 + □ = 20 | 16 | Watch for: guess-and-check without adjusting |
| 2 | 5 + □ = 20 | 15 | Check: treating the equals sign as 'the answer comes next' |
| 3 | 2 + □ = 20 | 18 | Look at: writing the whole equation as the answer |
| 4 | 15 + □ = 20 | 5 | Ask about: using the same operation instead of the inverse |
| 5 | 18 + □ = 20 | 2 | Watch for: ignoring the box and computing the two visible numbers |
| 6 | 10 + □ = 20 | 10 | Check: freezing when the box is first |
Work 15 + □ = 20 out loud as a demonstration. It gives 5. Children who hear 15 + □ = 20 reasoned through once at the start of a session make noticeably fewer errors in the ten minutes that follow.
Try 2 + □ = 20 together before letting the child start alone. The answer to 2 + □ = 20 is 18, and talking through how it was reached takes about a minute and saves several minutes of guessing later in the sheet.
The answer key is generated with the worksheet, not written by hand, so it always matches the sheet in front of you — 4 + □ = 20 is listed as 16, 5 + □ = 20 as 15, and so on down the page.
Keys are produced by the same code that produces the questions, so a mismatch between question and answer is not possible on these sheets.
To save these number bonds to 20 worksheets instead of printing them, choose 'Save as PDF' in the printer list. You get the worksheet and the answer key as a single PDF, on Letter or A4 depending on your printer settings.
If the problems look cramped, drop the count from 20 to a smaller number before printing these number bonds to 20 worksheets; the font stays the same size and the spacing opens up.
When these become easy, move along the related links below rather than adding more problems to the same sheet.
→ Free Missing Number Worksheets
Free printable math worksheets from a working math academy director in Korea. Problems come from our own code, not from a published workbook. Every sheet prints on US Letter or A4 with its answer key. All worksheets · Worksheet generator · Korean site
Last updated: 2026-09-13