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。

你看一下《韦氏高阶》的二个例句：

The trip takes a mere two hours. [=the trip takes only two hours]

You can own this car for a mere 20 dollars a week.

作为形容词，mere意为量很少的、微不足道的。以上二个词典例句中，数词two 后接时间名词hours必须用复数形式，但在说话人的心目中two hours被看作一个整体，是a short time, 很短的一段时间；同理，20 dollars 在说话人心目中被看作是一个整体，是微薄的薪水—— a small salary.  这就是说话人用不定冠词的心理因素。这和Two hours is a short time. 谓语动词用单数形式是一个道理。如果复数名词前没有数量限定词的话，mere前是不加不定冠词的。例如词典例句：

These mysteries can't be solved by mere mortals like us.

你的句子与上述二个词典例句是同一个类型，twenty miles被视为a short distance, 是一个整体，所以用了不定冠词。如果不用不定冠词，则应删去数词，mere 直接修饰复数名词miles。即 The next village was mere miles away. 或者改用副词 only： The next village was only twenty miles away.

1.这类MERE/SCANT/full等在这里都是看成修饰名词词组的修饰语，注意不是名词中心词的修饰语。A mere twenty miles; A scant (three days) ago=three scant days ago. 章振邦先生貌似没说为啥用程度修饰语修饰名词词组一定加A，我个人认为这里把TWENTY MILES看成一个整体了，比如 A mere (two miles) isn’t too far to walk.这里用单数就说明看成一个整体。

2. The next village was twenty miles away. 这类就是明显的Twenty miles作程度修饰语(传统语法叫做程度状语)修饰副词Away.现代语法把TWENTY MILES看成词组级别修饰语。