题目内容：

1、I spent half an hour（working on）this difficult math problem 括号里为何填working on 而不是to wo

优质解答

spend 有两个用法：spend some time （in）doing sth.spend some time on sth.
It takes sb some time to do sth.
上面的区别就可以很清楚说明如何填这个空了.