例句：. Nancy is so poor that even fifty dollars is a big sum to her.
fifty dollars 虽为复数形式，但它被视作一个整体，即一笔钱，因此，谓语动词应是单数形式。
长度、重量、时间等名称多作单数看待：
Fifty dollars (= The sum of fifty dollars) is not a large sum.
Three yards (= The length of three yards) is not enough.
Six months (= The period of six months) is too short a time to learn anything.
Five kilograms (= The weight of five kilograms) is enough.
这类带数词的名词前还可加一个指示词：
One short seven days is like seven years.
That three dollars (= That capital of three dollars) was the beginning of his career.
That second six months was happy indeed.
Every ten minutes is a trial.
A fortnight is not long enough to do the job.
This two kilograms of meat is enough for ten persons.
An estimated two thousand persons was killed in this hurricane.
A good ten miles is the distance we have climbed.
A happy three days has passed like a dream.
Such a weary six years was to me a terrible dream.
Ten apples (= Such a number of apples) is enough.
Two thousand immigrants is the quota for us this year.
如对组成部分加以强调，动词也可用复数形式：
Fifty dollars are too expensive.
Three yards cost thirty dollars.
