Key takeaway
What percent of Y is X = (X ÷ Y) × 100. So what percent of 200 is 50? (50 ÷ 200) × 100 = 25%. Divide the part by the whole, then scale to 100 — the same part-over-whole form, just phrased with the whole named first.
Scope and assumptions
General arithmetic. The 'whole' must be the full total in your context. This site is informational and not official.
Short answer: What percent of Y is X = (X ÷ Y) × 100. So what percent of 200 is 50? (50 ÷ 200) × 100 = 25%. Divide the part by the whole, then scale to 100 — the same part-over-whole form, just phrased with the whole named first. This page focuses on the wording where the whole (Y) leads the sentence, common in surveys and polls; for the "X is what percent of Y" form, see X Is What Percent of Y, and for "A of B" notation, see What Percentage Is A of B.
Quick navigation
- X is what % of Y → x-is-what-percent-of-y
- % of total → percentage-of-a-total
- The formula → percentage-formula
- The calculator → Percentage Calculator
1. The method
% = (X ÷ Y) × 100
X is the piece you are asking about; Y is the whole it is part of.
2. Worked examples
| What percent of Y is X | (X÷Y)×100 | Result |
|---|---|---|
| 200 is 50 | (50÷200)×100 | 25% |
| 50 is 20 | (20÷50)×100 | 40% |
| 80 is 40 | (40÷80)×100 | 50% |
| 100 is 25 | (25÷100)×100 | 25% |
3. Common pairs to know
- Half: X is 50% of Y when X = Y/2.
- Quarter: X is 25% of Y when X = Y/4.
- Tenth: X is 10% of Y when X = Y/10.
- Same: X is 100% of Y when X = Y.
4. A data example
In a survey of 200 people, 50 chose option A.
What percent of 200 is 50? (50 ÷ 200) × 100 = 25%
So option A drew 25% of respondents.
5. Common mistakes
- Dividing Y by X — that flips the question and the answer (200÷50 = 400%).
- Forgetting ×100 — 50÷200 = 0.25, which is 25% only after ×100.
- Using a subtotal as Y — Y must be the full whole.
- Mixing units — X and Y must be in the same unit.
6. Common pairings to know
| Part | Whole | Percent |
|---|---|---|
| 1 | 4 | 25% |
| 1 | 2 | 50% |
| 3 | 4 | 75% |
| 1 | 10 | 10% |
| 1 | 5 | 20% |
Memorize these and "what percent of Y is X" becomes quick: 20 is what % of 100? 20%. 20 is what % of 50? 40%. The pairing tells you instantly whether your computed percent is in the right ballpark before you trust it.
7. The denominator is the whole
In "what percent of Y is X", Y is the denominator because it is the whole the part X belongs to. Writing X on top and Y on the bottom is not optional — it is the definition of the question. A 50 of 200 is 25% because 50 sits a quarter of the way up to 200, not because 200 is somehow "of" 50. When the wording puts "of" between two numbers, the second one is the base; keep that rule and the division writes itself, and the percent you compute is the share of the correct whole rather than an inverted fraction that looks plausible but is wrong.
8. State Y, then divide
In this wording Y is named first for a reason: it is the whole. Say "Y is the base" before you touch the numbers, then put X over Y and scale to 100. A 20 of 50 is 20/50 = 40% because 50 is the base, not because 20 leads. The order of the words is the order of the division, and trusting the sentence removes the inversion that produces a wrong, often inflated, percent. Name the base aloud and the fraction writes itself, leaving no room to flip the numbers by accident.
FAQ
Q: What percent of Y is X? (X ÷ Y) × 100. What percent of 200 is 50? (50 ÷ 200) × 100 = 25%.
Q: What percent of 50 is 20? (20 ÷ 50) × 100 = 40%.
Q: How do I know which number is Y? Y is the whole — the total the part X belongs to.
Q: Why did I get the wrong percent? You likely divided Y by X. Keep X (the part) on top, Y (the whole) on the bottom.
Sources: Khan Academy percentage basics; Maths is Fun — Introduction to Percentages.
Disclaimer: General arithmetic. The 'whole' must be the full total in your context. This site is informational and not official.
Sources and references
- Khan Academy percentage basicshttps://www.khanacademy.org/math/pre-algebra/x31947de81e3f6453:percentages
- Maths is Fun — Introduction to Percentageshttps://www.mathsisfun.com/percentage.html