The word comes from the Malay for reckoning or estimating, and generations of Malaysian schoolchildren know it as the part of the maths lesson where the teacher calls out problems and you answer within seconds. It has been in the primary curriculum for decades and it survives the arrival of the smartphone for a reason.

Mental arithmetic is not about replacing a calculator. It is about knowing roughly what the answer should be before the calculator produces one, which is what stops a misplaced decimal point going unnoticed. There is more on the tradition and where it came from at cerdikcerdik.com/blog/what-is-congak.

Easy is addition and subtraction with numbers up to 20, on a 15-second timer. Medium adds multiplication and works with numbers to 50 and times tables to 12. Hard brings in division, numbers to 100, and gives you 18 seconds.

Pro pushes the range to 200 with tables to 20 and allows 20 seconds. Genius works with numbers up to 500, multiplication from 11 to 20, and 25 seconds a question. Every level gives you three questions, and the timer grows with the difficulty so the harder levels are harder arithmetic rather than merely less time.

Monday and Tuesday run three levels, which is nine questions and about two minutes. Wednesday through Friday add a fourth level, and the weekend runs all five — fifteen questions in total.

The shape is deliberate. School nights are short and a challenge that takes twenty minutes does not get done; a two-minute one does. By the weekend there is time for the levels that actually stretch you. Building the habit first and the difficulty second is the way round that works.

Bridging through ten is the foundation: 8 + 7 becomes 8 + 2 + 5, because getting to a round number and continuing is faster than counting on. Compensation handles awkward numbers — 49 + 36 is easier as 50 + 36 − 1, and 98 × 4 as 100 × 4 − 8.

For multiplication, break one number apart: 14 × 6 is 10 × 6 plus 4 × 6. Doubles and near-doubles cover a surprising amount of addition, since 7 + 8 is just double 7 plus one. For division, ask how many groups rather than reciting a table backwards — 72 ÷ 8 is 'how many eights in 72', which most people find faster than hunting through the eight times table.

None of these are tricks. They are the standard mental strategies taught in the KSSR syllabus, and the reason speed follows from them is that each one turns a hard sum into an easy one plus an adjustment.

Fifteen to twenty-five seconds sounds tight, and it is meant to. Given unlimited time most people fall back on counting, which produces the right answer while teaching nothing. The clock forces you to commit to a method.

It also mirrors the conditions the skill is actually needed in — a school test, a market stall, splitting a bill. If you find yourself timing out on a level, that is the level to practise, not the one to avoid. For the same arithmetic with less pressure, Number Pop at cerdikcerdik.com/games/number-pop puts the answer in front of you to choose from.