作文中的数字能不能不用英文字母表达，而直接用数字。
比如：1/100, 2.5 等。

I think, you can do alternative,either figures like 2, 3 or their actual pronounciations like two, three, thirds. Due to different situations and expression variesity, I think the former is formal and the latter is casual. However, if you check out GRE math section, you will find full of figures instead of their original pronounciations.

Comments need a further investigation!!!