The exam calculator rewards familiarity, not just arithmetic skill. Most time lost on calculator questions comes from a handful of predictable mistakes: misreading what % does, missing an operator swap, or losing a stored value at the wrong moment. This article works through those cases so you can recognise them before they cost you time in the exam. Try each example on the CAT calculator as you read.

Percentages: the base is easy to get wrong

The most common mistake is assuming % always means “divide by 100.” It doesn’t — it depends on the operator pending when you press it.

Mistake: treating 100 + 10 % as “add 10, then take a percentage.”

What actually happens: after + or -, the calculator takes the percentage of the expression already on the left. 100 + 10 % = gives 110, because 10 % here means “10% of 100,” not the number 10.

A subtler version of this catches people who have otherwise learned the rule: the “left expression” is the running total, not just the last number typed.

100 + 20 + 10 % = gives 132.

The running total before the percentage key is 120. 10% of that is 12, and 120 + 12 = 132 — not 100 + 20 + 10.

If you’re chaining several additions before applying a percentage, pause and total them mentally first, so the result doesn’t surprise you.

After multiplication or division, % behaves completely differently. It divides the just-entered operand by 100 instead of taking a percentage of a running total:

100 × 10 % = gives 10 (that’s 100 × 0.1). 100 ÷ 10 % = gives 1000 (that’s 100 ÷ 0.1).

And when there’s no pending operator at all, % is the simple case: it just divides the display by 100, so 10 % gives 0.1.

The practical rule: before pressing %, check which operator is pending. Addition or subtraction means “percentage of the left side.” Multiplication or division means “percentage of what I just typed.” Try both families of examples on the calculator’s percentage rules section to build the habit.

Operator precedence: don’t calculate left to right by default

The calculator applies standard precedence — multiplication and division resolve before addition and subtraction — even though you enter keys strictly in sequence.

Mistake: assuming 2 + 3 × 4 = evaluates left to right as (2 + 3) × 4, which would give 20.

What actually happens: 3 × 4 resolves first, so the answer is 14.

When operators share the same precedence, they run left to right instead: 8 ÷ 2 × 2 = gives 8, because 8 ÷ 2 resolves before × 2 is applied, not because division is special-cased over multiplication.

If a question chains several operators, it can help to silently group the multiplication and division terms first, the way you would on paper, rather than trusting the order you typed them in.

Recovering from a mis-pressed operator

A frequent slip is pressing the wrong operator and worrying that the entry is now corrupted. It isn’t — pressing a new operator before entering the next number simply replaces the pending one.

5 + × 2 = gives 10, not an error. The + never fires; × replaces it before a second operand is supplied.

This means a mis-press costs nothing as long as you catch it before typing the next number. Once a digit follows the operator, though, that operator is locked in for the calculation — so correct an operator immediately, not after you’ve already started the next number.

Memory keys: the indicator is the source of truth

The M indicator lights up whenever memory holds a value — including a stored zero — and only clears when you press MC. A common mistake is pressing C expecting a fresh start to also clear memory; C only clears the current calculation, so a value stored earlier is still sitting in memory and will affect the next MR, M+, or M− you press.

Store 4 with MS, then compute 2 + MR =. The result is 6 — the recalled 4 is a supplied operand, just like typing it directly.

Before starting a new question that doesn’t need the stored value, get in the habit of pressing MC explicitly rather than relying on C to reset everything.

Errors don’t need a reload to fix

Dividing by zero or taking the square root of a negative number shows Error. The instinct to reload or restart the question wastes time — pressing a digit, ., C, or MR recovers cleanly, and memory is left untouched by the error. MS, M+, and M− are ignored while Error is showing, which protects memory from picking up an invalid value; it isn’t a bug if those keys seem unresponsive in that moment.

Practise the sequence, not just the answer

Reading worked examples helps you recognise the rules, but the exam rewards speed under pressure. Work through these examples directly on the CAT calculator — typing the exact key sequence, including the mis-press recovery in the operator-replacement example — builds the muscle memory that reading alone doesn’t.