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Mental math for adults: what actually helps

October 5, 2026 · 5 min read

The cashier says $17.80, you're holding a twenty, and for a moment your head goes blank. It's not that you can't do it. You just haven't had to in a long time.

"Forgotten" is the wrong word

Most adults who say they can't do arithmetic anymore haven't lost it. They've outsourced it. The calculator lives in the phone, prices are on the screen, the till works out the change. What's missing is less the knowledge than the grip: how do I even start when I have no pen?

That's where it helps to know a few routes that are easier in your head than what we learned on paper at school. On paper you work right to left, ones first. In your head that's clumsy, because you have to hold intermediate results you would normally have written down.

Route 1: Step to the round number

98 + 47. Instead of adding 8 and 7, take two from the 47 and give them to the 98: 100 + 45 = 145. It works backwards too. 63 − 28 becomes 63 − 30 + 2 = 35. You solve the clean problem first and correct for the small remainder at the end.

Route 2: Left to right

46 + 37. Tens first: 40 + 30 = 70. Then the ones: 6 + 7 = 13. Together 83. It feels wrong at first because you learned it the other way round. But it's also how you'd say the answer out loud: size first, then detail. If you only want to know whether the shop will cost about 80, you're done after step one.

Route 3: Break it up when multiplying

6 × 19 is unpleasant. 6 × 20 isn't. So 120 minus 6 is 114. Or 15 × 14: first 15 × 10 = 150, then 15 × 4 = 60, together 210. The rule behind it is boring but powerful: you may split a number any way you like, as long as you put everything back together at the end.

Route 4: 25 is a quarter of 100

24 × 25 looks hard. It isn't. Four times 25 is 100, so you divide 24 by 4 and tack on two zeros: 600. The same idea works for 50 (half of 100) and 5 (half of 10). Someone with a handful of such anchors in their head doesn't calculate faster, they calculate shorter.

Route 5: Dollars and cents separately

Six rolls at 65 cents? Six times 60 cents is $3.60, plus six times 5 cents is 30 cents: $3.90. With prices it almost always pays to split the whole into a smooth part and an awkward remainder.

Six problems to try

No pen, no phone. Give each one half a minute.

1. 98 + 47
2. 63 − 28
3. 6 × 19
4. 15 × 14
5. 24 × 25
6. 6 × $4.50

How often, how long, and what it gets you

A word on honesty. Practising mental math makes you better at mental math. That's well established and not surprising. Whether it improves your thinking in general is a different question, and the research on transfer is rather reserved (more in Does brain training work?). So we promise you nothing, except that you may stay a bit calmer at the till.

Five minutes a day is plenty. More important than duration is practising one route a week until it feels natural, rather than starting all five at once. And don't time yourself at the beginning. Speed arrives on its own once the route sits. It doesn't work the other way round.

If you look at prices anyway, you can attach the practice there: roughly what is this shop going to cost before the till says so? That's the smallest possible exercise, and it's there every single day. A good companion: Percentages in everyday life.

Where it turns up in daily life

Hardly any of us will ever need to work out 24 times 25. What comes up constantly, though, are small estimates with money and time. Is there enough cash in my wallet for the drinks? What time is it if the train is fifty minutes late and I was due at 2:25? How much flour per person if the recipe is for four and we are six?

That last kind, scaling quantities, is a good test. Four portions become six? You need half again: halve the amount in the recipe and add it on top. If you stumble for a second, you notice it isn't about the numbers but about the question of where to start.

One practical rule: when you can't see a problem right away, imagine it first in a smooth version. Instead of $17.80 you think $18. Instead of $3.95 a pound you think $4. You solve the smooth problem, and then you correct the small deviation. That doesn't make the calculation sloppier, it makes it possible in the first place. You can still get the exact figure afterwards if you need it. Mostly you won't.