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One fact family
If 238 + 157 = 395, then 157 + 238 = 395 too. Subtraction recovers the parts: 395 − 238 = 157 and 395 − 157 = 238. Addition and subtraction are inverse operations.
A missing part or a missing whole
In □ + 184 = 520, find a part: 520 − 184 = 336. In □ − 184 = 336, find the starting whole: 336 + 184 = 520. Look at the position of the blank before calculating.
Checking is not repeating the same error
If someone writes 612 − 248 = 374, check by adding: 374 + 248 = 622, not 612. The correct difference is 364. Now 364 + 248 = 612, so the check succeeds.
Worked examples
□ + 275 = 643 → □ = 368
Subtract the known part from the whole. Check 368 + 275 = 643.
820 − □ = 465 → □ = 355
Find how much was removed: 820 − 465 = 355. Check 820 − 355 = 465.
Try it yourself
Your turn
Type the missing word, number, or sign. Spaces in large numbers are allowed. Try first, then open the explanation.
Write four related equations using 172, 286, and 458.
See an example answer
172 + 286 = 458; 286 + 172 = 458; 458 − 172 = 286; 458 − 286 = 172.
Check your knowledge
Choose one answer for each question. You can revise your answers and check again.
Answer key
- 287. Subtracting one part leaves the other.
- 360 + 125. The starting whole is the remainder plus the removed part.
- 375. 590 − 215 = 375.
- No, the difference is 224. 234 + 178 = 412; 224 + 178 = 402.
- 383 − 129 = 254. The whole minus one part gives the other.