翻译以下句子为英语：容我提示一下：1在写作文的时候百分比比如30要写成thirty percent of 2在频率是12的时候使用once或twice在3及以上时使用n times3percent是可数名词单复数同形4How come大意为怎么回事？

Let me give you a hint:

1. When writing an essay, percentages like "30%" should be written as "thirty percent of ..."
2. Use "once" or "twice" when the frequency is 1 or 2, and use "n times" when it is 3 or more.
3. "Percent" is a countable noun and remains the same in both singular and plural forms.
4. "How come?" roughly means "What's the matter?" or "Why is that?"