How the % key behaves (percent of, +%, −%)
Percent is easiest when you think “percent of a base”. The same percent can mean “part of” or a “relative change”, depending on context.
In everyday calculations, you usually mean one of these three things: (1) “p% of base”, (2) “base increased by p%”, or (3) “base decreased by p%”.
A good calculator helps by interpreting % relative to the current base in addition/subtraction contexts. For example, “200 + 10%” is typically treated as “200 + (10% of 200)”.
When results look surprising, check what your “base” is. Percent is always applied to some base value — the key is identifying which one.
- 200 + 10% = 220
- 200 − 10% = 180
- 10% of 200 = 20
Shortcut: +p% means multiply by (1 + p/100)
Always ask: “p% of what base?”
Parentheses + order of operations
Multiplication/division happen before addition/subtraction — parentheses override grouping.
Most calculators follow standard precedence rules: parentheses first, then multiplication/division, then addition/subtraction. (Some also have powers, but this basic guide focuses on everyday arithmetic.)
That’s why 2 + 3 × 4 becomes 14: it’s 2 + (3 × 4). If you want (2 + 3) × 4, you must add parentheses.
When you’re typing longer expressions, adding parentheses often prevents mistakes and makes your intent obvious.
- 2 + 3 × 4 = 14
- (2 + 3) × 4 = 20
Parentheses are the safest way to control grouping.
Repeat “=” to apply the last operation again
Many calculators repeat the last binary operation when you press “=” again — great for quick increments and repeated discounts.
A common physical-calculator behavior is: after you compute A op B = result, pressing “=” again repeats “op B” using the previous result as the new A.
Example: 5 + 2 = 7, then “=” again gives 9, then 11, and so on. This is handy for repeated additions (like counting by 5s) or repeated multiplications.
If you ever get a surprising chain of results, it’s usually because the calculator is repeating the last operation. Clearing the operation (or using AC) resets that state.
- 5 + 2 = 7
- Press “=” again → 9
- Press “=” again → 11
Repeat “=” applies the last op with the same B value.
Quick Reference
| Pattern | Meaning | Example |
|---|---|---|
| p% of base | (p/100) × base | 10% of 200 = 20 |
| base + p% | base × (1 + p/100) | 200 + 10% = 220 |
| base − p% | base × (1 − p/100) | 200 − 10% = 180 |
| a + b × c | b × c first | 2 + 3 × 4 = 14 |
| (a + b) × c | parentheses first | (2 + 3) × 4 = 20 |
FAQs
Why does 200 + 10% become 220?
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Why does 200 + 10% become 220?
▾Because +10% is treated as “add 10% of the current base (200)”. 10% of 200 is 20, so 200 + 20 = 220.
Why does 2 + 3 × 4 equal 14 instead of 20?
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Why does 2 + 3 × 4 equal 14 instead of 20?
▾Because multiplication happens before addition. It’s evaluated as 2 + (3 × 4). Use parentheses if you want (2 + 3) × 4.
What does AC vs C mean?
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What does AC vs C mean?
▾AC typically clears everything (expression + memory of the last operation). C typically clears just the current entry/value.
What does the backspace (⌫) key do?
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What does the backspace (⌫) key do?
▾It deletes the last character or digit, letting you correct input without clearing the whole expression.
Why does pressing “=” again change the result?
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Why does pressing “=” again change the result?
▾Many calculators repeat the last operation with the same second operand. For example, after 5 + 2 = 7, pressing “=” again gives 9 (adds 2 again).
Do I always need parentheses?
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Do I always need parentheses?
▾Not always, but parentheses remove ambiguity. If an expression mixes +/− with ×/÷, adding parentheses often prevents mistakes.