请问下面这句英语中mere 前面为什么加一个不定冠词a ,后面的名词词组的中心词miles明明是个复数吗.
这是新概念英语第三册39课里的一句话
Glancing at his map,he informed us that the next village was a mere twenty miles away.例如：词典里的例句 Mere words won't help.（光说不练,无济于事）,这里就没有冠词.