a mere twenty miles 为什么前面加 a

1. mere 是一个强调形容词，强调数量少，用在数词之前。尽管被数词修饰的名词用复数，也要使用不定冠词。即把后面的成分作为一个整体看待。

It took her a mere 20 minutes to win.

她只花了20分钟就赢了。

《牛津词典》

Sixty percent of teachers are women, but a mere five percent of women are principals.

60％的教师是女性，但只有5％的女性是校长。

《柯林斯英汉双解大词典》

It took her a mere 20 minutes to dispose of her opponent.

她仅用了20分钟就击败了对手。

《牛津词典》

The American came from behind to snatch victory by a mere eight seconds.

美国队后来居上，仅以8秒钟的优势赢得了胜利。

《柯林斯英汉双解大词典》

2. 形容词 estimated, good, full, clear, cool等，也是此用法。

An estimated 1,600 vessels pass through the strait annually.

估计每年有1600艘船经过这个海峡。

《柯林斯英汉双解大词典》

There are an estimated 90,000 gangsters in the country.

这个国家估计有9万名歹徒。

《柯林斯英汉双解大词典》

By the end of May nearly 100 of an estimated 2 000 political prisoners had been freed.

至五月底，估计2000名政治犯中有近100人已经获释。

《牛津高阶英汉双解词典》

An estimated seven million people are at risk of starvation

估计有700万人面临着饿死的危险。

《柯林斯高阶英汉双解学习词典》

网友后面的两个例句正确。