如何用英文口頭解釋一段計算，而不像一台計算機
某位學生在幫朋友家教。朋友問：「If a coat costs forty dollars and there's a twenty-five percent discount, what's the price?」學生知道答案是 30 美元，於是脫口而出：「Forty times zero point seven five equals thirty.」朋友禮貌地點點頭，但一臉茫然。數學沒錯，但故事不見了。把計算說出來，並不等於把它算出來。要做的是帶著對方走過每一個步驟，就像在熟悉的街上幫人指路。
為什麼這件事重要
你會比想像中更常需要解釋計算——上班討論預算、上課對答案、開會描述圖表、面試時走過 case study、口說考試的整合題要你總結資料。許多學習者會在這個時候卡住，因為他們只學過怎麼「做」這個數學，沒學過怎麼「敘述」它。母語者用一小組指路用語——first、then、next、which gives us、so、that comes out to——把一個算式變成一個故事。一旦學會這套工具，幾乎任何計算你都能從容地用嘴巴帶過去。
這篇跟 英文怎麼念小數、分數與比例：再也不卡住 是天生一對，因為一旦開始口頭講解數學，就得知道小數與分數該怎麼念出來。
用法規則
一段口頭計算有三個部分：setup（鋪陳）、steps（步驟）、result（結果）。
Setup 介紹題目。
- 「Let's say the coat costs forty dollars.」
- 「Suppose the population is two hundred thousand.」
- 「Imagine you have a budget of five thousand.」
- 「Take a sales total of one hundred thousand.」
Steps 一段一段帶過算式。
- 「First, I take twenty-five percent of forty, which is ten.」
- 「Then, I subtract that from forty.」
- 「Next, I multiply by the number of months.」
- 「After that, I divide by twelve to get the monthly figure.」
- 「Now I add the tax.」
Result 自信地把答案落地。
- 「That gives us thirty dollars.」
- 「That comes out to about eight percent.」
- 「Which works out to roughly fifteen hundred.」
- 「So the answer is seventy-five.」
- 「And there you have it — thirty dollars.」
英文裡的運算字眼都很短，請把它們當成真正的字念，而不是符號。
- Plus 或 add 代表 +
- Minus 或 subtract 代表 −
- Times、multiplied by，或只說 by 代表 ×
- Divided by 或 over 代表 ÷
- Equals、is、gives 或 comes out to 代表 =
- Squared 代表 ²，cubed 代表 ³，to the power of 代表更高次方
- Out of 用在像 18 out of 30 的分數
- To 用在 3 to 1 這類比例
- Percent of 用在 20% of 50
也可以用 basically、the way I'd do this，或 here's the trick 預先告訴聽者結構。聽者很愛這些字，因為它們在說「跟著我，不會痛」。
錯誤／自然／為什麼
| 錯誤 | 自然 | 為什麼 |
|---|---|---|
| Forty times zero point seven five equals thirty. | First, I take 25 percent of 40, which is 10. Then I subtract that from 40, so the answer is 30. | 第一句只點出算式，沒講步驟；第二句把邏輯講出來。 |
| First I do calculation, then I divide. | First I work out the gross figure, then I divide by twelve. | 英文動詞用 work out 或 calculate；do calculation 不自然。 |
| Twenty over hundred is point two. | Twenty divided by one hundred is zero point two. | Over 在某些情境可行，但敘述計算時 divided by 比較清楚。 |
| 40 times 0.75 give us 30. | 40 times 0.75 gives us 30. | 主詞是單數時 gives 要加 -s。 |
| That's coming out to 30. | That comes out to 30. | 固定片語是 that comes out to，用簡單現在式，不用進行式。 |
| First, second, third, I have the result. | First, then, finally, I have the result. | 口語的算式步驟通常是 first / then / next / finally，不是 first / second / third。 |
| Three power of two is nine. | Three to the power of two is nine.（或 three squared） | 固定形是 to the power of。 |
| Twenty out from thirty | Twenty out of thirty | 分數用 out of，不是 out from。 |
| Take the half from the total. | Take half of the total.（或 subtract half from the total） | Take the half from 把兩種用法混在一起。 |
常見情境
簡單折扣。「Let's say the jacket is one hundred dollars and the discount is twenty percent. First, I work out twenty percent of one hundred, which is twenty. Then I subtract that from one hundred, so the price comes out to eighty dollars.」注意說話者不會直接宣布「答案是 80」，他是一步一步堆上去的。
分帳。「Take the total of one hundred and twenty dollars. Divide that by four people, which gives us thirty each. If we want to add tip, say fifteen percent, that's eighteen on top, so each person owes about thirty-four fifty.」說話者用 which gives us、say 與 so 把運算串起來，每一步都有指路詞。
百分比上升。「Imagine your salary was sixty thousand and you got a ten percent raise. First, ten percent of sixty thousand is six thousand. Then you add that to the original, and there you have it — sixty-six thousand.」這就是 increase by 模式的對話版。
期中期末加權平均。「The way I'd do this is: take her midterm score, multiply it by point four, then take her final, multiply by point six, and finally add them together. So, eighty times point four is thirty-two, ninety times point six is fifty-four, which adds up to eighty-six.」聽者就算自己不會算，也能跟著走過這段算式。
估算油錢。「Suppose the trip is three hundred miles and the car gets thirty miles per gallon. First, divide three hundred by thirty, which is ten gallons. At four dollars a gallon, that comes out to forty dollars for the trip.」注意說話者用 at 帶出單價——對話中的常見做法。
簡報描述圖表。「If you look at the second quarter, revenue rose from two million to two point four million. That's an increase of four hundred thousand, or roughly twenty percent. So we're up year-over-year by about a fifth.」主講者把粗略運算疊在圖表上，帶著觀眾走過數字代表的意義。
常見錯誤
- 把算式當成原始符號念出：「Forty times zero point seven five equals thirty」，沒有鋪陳也沒有指路詞。加上 first、then 與一句簡短的解釋。
- 跳過 setup。聽者需要知道這些數字代表什麼，Let's say、suppose 或 imagine 給他們一個框架。
- 忘了讓結果落地。所有步驟講完後，要用 that gives us、which comes out to 或 so the answer is 收尾，否則聽者會懸在半空中。
- 把小數念成複合數字（「zero point seventy-five」而非「zero point seven five」）。小數點之後要一個一個念。
- 用 do 表達運算：do the multiplication、do the calculation。應該用 work out、calculate、figure out，或直接用運算動詞：multiply、divide。
- 把 out of 跟 out from 搞混，正確是 18 out of 30。
- 動詞需要 -s 時忘了加：應該是 which gives us，不是 which give us。
- 過度使用 so。當指路詞很棒，但每句都用 so 開頭會顯得重複，可以混搭 which、then 與 and。
- 結果省略單位。「Thirty」聽起來孤伶伶的；「thirty dollars」或「thirty percent」才會告訴聽者答案的種類。
小練習
每題請大聲走過一次，用「setup + 步驟用語 + 收尾用語」三段式講出來。
- A laptop costs $1,200 with a 15% discount. What is the final price?
- A class has 40 students. 30% are absent. How many are present?
- Sales were $5,000 in Q1 and rose by 20% in Q2. What was the Q2 total?
- You drive 240 miles in 4 hours. What is the average speed?
- A recipe makes 24 cookies. You want to make a third of that batch. How many cookies?
總結
口頭講解計算是一半數學、一半說故事。Setup 鋪陳問題（「Let's say…」），步驟用 first、then、next、which gives us 串起來，收尾用 so the answer is 或 that comes out to 把答案落地。用自然的運算字眼——plus、times、divided by、out of——並像導遊一樣指路。一旦這些片語變成直覺，任何計算你都能從容講出來，不管是在考試、在會議，還是在朋友家吃飯時被臨時要求分帳。
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