Concept 6 of 7
The Rewind Button: Checking Your Work
Today's big idea
Undo your answer with the opposite operation and see if you land where you started.
If 7,500 − 2,300 = 5,200, rewind: 5,200 + 2,300 should give 7,500.
How it works
👀 What's going on?
You just solved 7,500 − 2,300 and got 5,200. Are you sure? Checking by doing the same sum again rarely helps, because you tend to repeat the same slip. There is a better way: rewind it.
💡 Here's the trick
Look at the picture above. Adding and subtracting are opposites. If 5 + 3 = 8 is true, then 8 − 3 must give back 5. Undoing what you just did should land you exactly where you started.
🪄 How to do it
- 1.Write down where you started and where you finished, as the picture above shows.
- 2.Do the opposite operation on your answer.
- 3.If you land on your starting number, the answer is right. If not, you have found a mistake.
✍️ Try it
A friend says 8,400 − 2,600 = 6,200. Rewind it and see. Does the check land back on 8,400?
Add the answer back: 6,200 + 2,600 = 8,800. That is not 8,400, so the answer is wrong. The correct answer is 5,800, and 5,800 + 2,600 does give 8,400. That is the whole point of rewinding. It catches the slip before anyone else sees it.
⚠️ Watch out
You might be thinking an addition rewinds the same way a subtraction does. Say you work out 4,200 + 1,300 = 5,500. Taking 5,500 away from 5,500 gives 0 and proves nothing. Take one part away instead, so 5,500 − 1,300 should land you back on 4,200.
🌟 Why this matters
Checking your own work is a habit, not a maths skill, and it pays off far outside maths. Strong problem solvers rewind almost without noticing.
Exam tip: A five-second check catches most careless slips, and careless slips cost more marks than hard questions do.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“I solved 9,400 − 3,600 and got 5,800. Is that right?”
- 1
Look at the answer
We say 9,400 − 3,600 = 5,800. Time to prove it.
- 2
Hit REWIND ⏪
Do the opposite — subtraction flips to addition.
- 3
Add it back
5,800 + 3,600 = 9,400
- 4
Lands at the start ✓
Matches the starting number! Answer was right.
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
The Rewind Button proves your maths — do the opposite and check it lands back on the start.
CHECK 1 OF 2
Quick check
✅ VERIFICATION
You worked out 4,000 + 1,600 = 5,600. How do you rewind that to check it?
⚡ Finish the activity above first to answer.
Keep going!
Up next: Estimate & Check
4,812 + 3,197 ≈ 5,000 + 3,000 = 8,000.