the thirties：三十到三十九這一段
一旦加上 the 和複數 -s，30 就不再是一個點，而是一整段 30 到 39。這一段可以掛在年份、年紀或溫度上，用法完全一樣。
aged 30 to 39 · number · A2
If you are in your thirties, your age is between 30 and 39 years old.
三十幾歲
核心畫面
Picture the count rolling over: twenty-nine, then a new ten opens and you land on 30. That one number is the whole word. Everything else happens when English takes 30 as the start of a stretch — 30, 31, 32 … 39 — and calls the whole stretch the thirties, a plural you can use for years, ages, or degrees.
可以想成數線上數過 29，翻過一個十位，落在 30。這個數字本身就是整個字。其他用法都是同一件事：英文把 30 當成…更多收起
可以想成數線上數過 29，翻過一個十位，落在 30。這個數字本身就是整個字。其他用法都是同一件事：英文把 30 當成一段的起點——30、31、32…一直到 39——再把整段叫作 the thirties（複數），年代、年紀、溫度都能這樣講。From the picture
Also in this word
the number 30 · number · A2
The number 30, which comes after twenty-nine and before thirty-one.
三十
原始詞典資料3 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
thirtynum/ˈθɜːti/
- 1
The cardinal number occurring after twenty-nine and before thirty-one, represented in Arabic numerals as 30.
- To conclude, here in tabular form is a summary of our ratings of espressos produced from thirty pod, capsule and disc blends.
thirtynoun/ˈθɜːti/
- 1
slangA rack of thirty beers.
- 2
broadcastingmediaA commercial lasting 30 seconds.
- […] instead of thirties [thirty-second commercials], they buy sixties, which cost twice as much. They spend more on filming it than they need to; they buy a lot of ads that are garbage and don't really say anything, just to fill up the space.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。