我用一句话概括这三个词组的含义：

Tom has a number of books, but he doesn’t exactly know the number of his books. Because he is poor at maths, he can’t add up the numbers of his books and his pens.

【翻译】汤姆有许多书，但他不能确切地知道书的数量。由于他数学不好，所以他无法把书和钢笔的数目加起来。

请你背诵这句话，并体会比较这三个词组的意思。

关于这三个词组作主语或主语修饰语时，谓语动词用单数还是复数，请注意红色部分：

1. A number of books are sold every day in this bookstore. 这家书店每天都有许多书被出售。

2. The number of his books is very large. 他的书的数量很大。

3. The numbers of all his books and pens add up to 120. 他所有的书和钢笔的总数加起来是120。