Mental math under pressureFree9 min read

The techniques that matter

Rounding with correction, factor shuffling, and the percentage tricks worth automating.

Case maths is not competition arithmetic. It is a small set of operations on large round numbers, done out loud without mistakes. Drill these five and you cover the overwhelming majority of what appears.

Round and correct: 47 x 21 becomes 47 x 20 = 940, plus 47 = 987. Faster and more reliable than long multiplication.
Shuffle factors: 25 x 48 becomes 100 x 12 = 1,200 by moving a 4 across. Look for 25, 50, 125 and pair them with their complements.
Work in units, not zeros: write $4.2bn as 4.2 and track the unit in the margin. Almost every arithmetic disaster in a case is a misplaced zero.
Percentages via fractions: 12.5% is one eighth, 16.7% is one sixth, 37.5% is three eighths. Dividing is faster than multiplying by a decimal.
Estimate before you compute: say 'this should land near 900' before starting. You will catch order-of-magnitude errors instantly.

Narrate the setup, not the arithmetic

Say the equation out loud, then go quiet and compute, then state the answer with its unit. Narrating each digit is slow and makes small slips sound catastrophic. Narrating the setup gets you credit even if the arithmetic wobbles.

Now put it into practice

Reading only gets you so far.

Drill these techniques against a clock