Mental Math · Subtraction · S2
Subtraction is really a question about distance: 92 − 34 asks 'how far is it from 34 up to 92?'. Instead of taking away, count up through friendly numbers and add the hops. This is exactly how you work out change.
See the hops
Add the hops: 6 + 50 + 2 = 58.
Watch the method
From 34, hop to 40 (+6), then to 90 (+50), then to 92 (+2). The hops add to 58 — that's the distance, and the answer.
Hop up to 40.
Hop up to 90.
Hop up to 92.
Add the hops — that distance is the answer.
Your turn
Use the method, then check. Stuck? Reveal the steps.
Taking away and finding the gap give the same answer. Counting up is easier because every hop lands on a friendly number, and you only ever add.
Why it works
It's the shopkeeper's method: from the price, count coins up to the cash given — that's your change.
80 − 35
To a friendly ten
35 → 40 is +5.
Up to 80
40 → 80 is +40.
Add the hops
5 + 40 = 45.
92 − 34
To 40
34 → 40 is +6.
To 90
40 → 90 is +50.
To 92
90 → 92 is +2.
Add
6 + 50 + 2 = 58.
503 − 248
To 250
248 → 250 is +2.
To 500
250 → 500 is +250.
To 503
500 → 503 is +3.
Add
2 + 250 + 3 = 255.
Counting up but forgetting to add all the hops.
The answer is the total of every hop: 6 + 50 + 2 = 58. Don't drop one.
Hopping past the target.
Stop exactly on the bigger number. The last hop is whatever is left to reach it.
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t₁ = 5, d = 3 and t1=5,d=3 both count. For a list of numbers, separate them with commas (e.g. 3, 10, 17).92 − 34 = ?
80 − 35 = ?
63 − 28 = ?
71 − 46 = ?
100 − 63 = ?
120 − 85 = ?
154 − 78 = ?
200 − 137 = ?
300 − 176 = ?
412 − 285 = ?
503 − 248 = ?
650 − 389 = ?