这个主要通过看be动词后面的成份来判断其主语是单数还是复数.
The population of China is 1.3 billion.be动词is后面是一个数字,所以很明显用第三人称单数形式；
Three fifths of the population in our school are women teachers.be动词后面是women teachers,很显然是复数,那么主语three fifths of the population也就倍当作是复数了.
希望对你有所帮助.