Question

A recent study shows that the number of middle-aged people dying as a result of being overweight

has doubled in 10 years in Britain不明白为什么不用a number of呢？

Answer 1

a number of, the number of 的区别

a number of:指的是"许多",谓语动词用复数
the number of:指的是"什么什么的数量",谓语动词用单数
楼主的题目中谓语动词用了第三人称单数shows。所以用the number of
例子:
A number of soldiers were detached to guard the building.
许多士兵被派去保护这座建筑物。
The number of tigers has reduced abruptly in the last fifty years.
在过去的五十年里，老虎的数目锐减。

追问

按你这样说好像翻译不出中文啊？
不行你试试

追答

可以啊，但是楼主不一定要按照他的意思来直接翻译。英语翻译是很灵活的
最近的一项研究表明，死于超重的中年人的数量越来越多