CAT Calculator

Usage guide

The CAT calculator reproduces the exam calculator's controls and quirks, including how it resolves percentages and the order it evaluates operators in. Read a section below, then try the example on the calculator above.

Controls

0–9 and .

Enter a number. Entry is capped at eight digits; the sign and decimal point don't count against the cap.

+ − ×÷

Choose an operator. Pressing a different operator before entering the next number replaces the pending one instead of stacking it, so 5 + × 2 = gives10, not an error.

=

Evaluate the expression. Repeated presses of = do nothing further — 5 + 2 = = = stays at7.

%

Apply a percentage. What it does depends on the pending operator — see percentage rules below.

C

Clear the current calculation. Memory is untouched, so a fresh start doesn't cost a stored value.

← (backspace)

Remove the last digit typed. It only edits digits you typed yourself — a computed result or a value recalled from memory is left alone.

±

Flip the sign of the current entry.

1/x

Replace the current entry with its reciprocal.

√

Replace the current entry with its square root. The result can be used as an operand, so 9 + 16 √ = gives13.

MS MR M+M− MC

Store, recall, add to, subtract from, and clear memory. TheM indicator lights up whenever memory holds a value — including a stored zero — and goes out only when MC is pressed.

Dividing by zero or taking the square root of a negative number shows Error. Memory keys are ignored while an error is showing, but you can always recover by pressing a digit,., C, or MR.

Percentage rules

The % key doesn't just divide by 100 — what it does depends on the operator pending when you press it.

1. After + or −, it takes a percentage of the expression to its left.100 + 10 % = gives 110, and100 − 10 % = gives 90 — the10 is treated as 10% of 100, not the literal number 10.
2. That left-hand base is the whole expression evaluated so far, not just the last number entered.100 + 20 + 10 % = gives 132: the running total is 120 before the percentage is applied, so 10% means 12, and120 + 12 = 132.
3. After × or ÷, it divides the entered operand by 100 instead.100 × 10 % = gives 10 (multiplying by 0.1), and 100 ÷ 10 % = gives1000 (dividing by 0.1).
4. After a multiplication, the percentage uses only the operand you just entered — not the running total.100 + 20 × 10 % = gives 102: the20 becomes 2 (10% of 20), and100 + 2 = 102.
5. With no pending operator, it simply divides the display by 100. 10 % gives 0.1.

Operator precedence

Multiplication and division resolve before addition and subtraction, the way they would on paper. 2 + 3 × 4 = gives 14, not 20, because the3 × 4 is evaluated first. Operators that share the same precedence run left to right instead, so8 ÷ 2 × 2 = gives 8.

Try these examples on the CAT calculator to see the order of evaluation for yourself.

Ready to go deeper? ReadCAT calculator tips and tricks for worked examples and common mistakes to avoid.