Common Math English Mistakes That Make Teachers Stop You Mid-Sentence
A student was describing a graph during a speaking practice session. He said, "Sales increased of twenty percents, from thirty to fifty percentage." His tutor raised a hand mid-sentence: "Hold on — three problems in one breath. Let me unpack them." That single sentence had a wrong preposition, a wrong plural, and the wrong word for the difference between two percentages. Math English is a minefield of small slips, and tutors hear the same ones again and again. The good news is: there are only about a dozen, and once you learn the fix, you can patch the whole pattern.
Why This Matters
Numbers are everywhere in real-world English — bills, schedules, weather reports, sports scores, lectures, salary talks, charts in presentations, grades, prices, and percentages on the news. Math sentences are also where a careful listener can spot an English learner the fastest. The grammar is short, the vocabulary is small, but the rules are unforgiving. A single slip — percents with an s, increase of instead of increase by, three point fourteen instead of three point one four — instantly stamps the sentence as non-native. The fixes are quick. The payoff is enormous, because once you stop tripping on numbers, the rest of your English sounds much more confident.
This article is the cleanup article in the series. If you want the deeper dives, see Five Dollars or Five-Dollar? How Numbers Become Adjectives in English for the hyphen-and-plural rule, and Percent, Percentage, and Percentage Points: Small Words, Big Mistakes for the chart-description trap.
The Pattern
A handful of mistakes account for the vast majority of math-English errors learners make. Here they are, grouped.
Wrong preposition. English uses by for the size of a change, to for the new value, of for fractions and percentages of a base, and per for rates. Mixing them is the most common slip.
- Increased by ten percent (size of the change).
- Increased to fifty (new value).
- Twenty percent of fifty (portion of a base).
- Sixty miles per hour (rate).
Wrong plural. Percent never takes -s when attached to a number ("twenty percent," not "twenty percents"). Hundred, thousand, million, and billion stay singular when used as numbers ("five hundred people," not "five hundreds people"). But when used as nouns meaning many, they take -s ("hundreds of people").
Wrong reading of decimals. Digits after the decimal point are read one at a time, not combined into a normal number. 3.14 is three point one four, not three point fourteen.
Wrong reading of fractions. Fractions use ordinal numbers in the denominator. 1/3 is one third or a third. 2/3 is two thirds (with -s because the top number is more than one). 1/2 is a half or one half, irregular.
Wrong vocabulary. Saying percentage when you mean percent, or percent when you mean percentage points, can flip a sentence's meaning. Calling a rough estimate a calculation, or using count when you mean number, slightly off-tilts the sentence.
Wrong hyphenation. In adjective form before a noun, number-unit phrases drop the -s and take a hyphen: a five-dollar coffee, a ten-minute break, a two-year-old child. In predicate or object form, the -s comes back: the coffee costs five dollars, the child is two years old.
Wrong word order. Quantifiers like each and apiece sit after the price (ten dollars each), not before. Every comes before the number-time phrase (every two hours), not after.
Wrong / Natural / Why
| Wrong | Natural | Why |
|---|---|---|
| Sales increased of 20 percent. | Sales increased by 20 percent. | English uses by for the size of a change with verbs of motion. |
| The price rose to 10%, from 8 to 10. | The price rose by 2 percentage points, from 8% to 10%. | The gap between two percentages is percentage points, not percent or to. |
| Twenty-five percents of the budget is gone. | Twenty-five percent of the budget is gone. | Percent has no plural -s when attached to a number. |
| It is a five-dollars coffee. | It is a five-dollar coffee. | Before a noun, the unit drops -s and is usually hyphenated. |
| Three point fourteen | Three point one four | Digits after the decimal point are read one at a time. |
| Two third of the audience | Two thirds of the audience | The plural -s is required when the top of the fraction is more than one. |
| The car goes 60 miles per a hour. | The car goes 60 miles per hour. | After per, the unit takes no article. |
| What percent of students passed? | What percentage of students passed? | With no specific number, use percentage. |
| Sixty miles per hours | Sixty miles per hour | The unit after per stays singular. |
| Take this pill every of four hours. | Take this pill every four hours. | Every is followed directly by a number-time phrase, no of. |
| The total is round 100 dollars. | The total is around 100 dollars. (or roughly 100) | Around and roughly exist; round is for round numbers (clean numbers like 100). |
| She is a two-years-old child. | She is a two-year-old child. | Before a noun, the descriptor loses -s and gains hyphens. |
| Nearly 32 applicants, over 30 | Over 30 applicants, around 32 | Nearly points below the number; 32 is above 30. |
Common Situations
Describing data in a presentation. "Sales rose by about fifteen percent to roughly four point six million, from four million last year." Three quick fixes baked in: by for the size, to for the new value, and from … to … to show both endpoints clearly. If a learner says "sales increased of fifteen percents to four point six millions," every single one of those mistakes is fixable with one rule from this article.
Reading a price aloud at the supermarket. "The bag of rice is a dollar fifty a pound." Three things going on at once: the casual a / an form for rates, the each-equivalent a pound, and the dollar phrasing without an awkward "per." A learner who says "one fifty per pound" is grammatically fine but slightly stiff; "a dollar fifty a pound" sounds local.
Telling someone your age. "I'm twenty-eight years old." If a learner says "I have twenty-eight years," that is the literal translation of how age works in many languages, and English does not do it. Age in English uses be, not have. "I am twenty-eight" or "I am twenty-eight years old" or, before a noun, "a twenty-eight-year-old learner."
Reporting a test result. "I got eighty out of one hundred, which is eighty percent." Notice out of for the raw score, percent for the percentage. Learners often say "I got eighty percent of one hundred" — which means I got eighty percent of one hundred, an unclear, almost philosophical statement. The natural form is eighty out of one hundred, or just eighty percent.
Splitting a meal. "The bill is forty-eight dollars, so let's just round up to fifty and split four ways — that's twelve fifty each." Lots of math-English packed into one sentence: rounding, splitting, each in the right position, four ways for split among four people. Learners often say "split for four people," which is understandable but slightly off; split four ways is the natural English idiom.
Quoting a salary. "I make about seventy thousand a year." Casual rate form (a year), natural rounding (about seventy thousand). A learner who says "I am earning seventy thousands per year" stacks two of our top mistakes: the unnecessary -s on thousand, and the slightly stiff per year instead of a year. Tutors hear this combo constantly.
Common Mistakes
- Using of after verbs of change. The correct preposition is by: rose by, fell by, increased by, decreased by. Reserve of for noun forms like an increase of ten percent.
- Adding -s to percent, hundred, thousand, million, billion when they are attached to numbers. Five hundred, not five hundreds. Ten percent, not ten percents. The -s comes back only when these words are used as nouns meaning many: hundreds of people.
- Reading decimals like compound numbers. Three point one four, not three point fourteen. Each digit gets its own breath.
- Mixing up percent and percentage and percentage points. Specific number → percent. No specific number → percentage. Gap between two percentages → percentage points.
- Leaving the -s on a unit before a noun: a five-dollars coffee, a ten-minutes break. Before a noun, the unit shortens.
- Forgetting the hyphen in compound number adjectives: a two and a half hour drive. The standard form is a two-and-a-half-hour drive.
- Mixing up nearly and over. Nearly always points below the target; over always points above.
- Saying I have twenty years for age. English uses be: I am twenty.
- Saying give and take instead of give or take. The fixed idiom uses or.
- Using to when you mean by (or vice versa). Increased by ten describes the size of the change. Increased to ten names the new value. They are almost never interchangeable.
- Treating data and sales as always-singular. In business and academic English, sales is usually plural ("sales are up"), and data is treated as plural in careful writing ("the data show").
Mini Practice
Find and fix one or two mistakes in each sentence.
- Our team grew of five members last quarter, from eight to thirteen.
- The bill is forty-two fifty, so let's round up to forty-five and pay fifteen dollars each one.
- He is a two-years-old boy who already reads big books.
- Inflation rose to three percentage points, from two percent to five percent.
- The recipe calls for two third of a cup of sugar and one and half teaspoons of salt.
Summary
Math English has a small set of repeat-offender mistakes: wrong prepositions (by / to / of / per), wrong plurals (percent / hundred / thousand), wrong decimals (digit by digit, not combined), wrong fractions (the plural -s on the denominator), wrong vocabulary (percent / percentage / percentage points), wrong hyphens (before a noun, drop the -s and add a hyphen), and wrong word order for each and every. Patch these one by one, practice them out loud, and your number sentences will stop pulling teachers' hands into the air mid-sentence — and instead, they will sound exactly like the English you have been working so hard to build.
Want to practice numbers, quantifiers, and units in real test sentences? Start practicing on ExamRift.