He had no difficulty working out the problem为什么是working
问题描述：
He had no difficulty working out the problem为什么是working
答
work out(解决)是固定搭配；have difficult (in) doing sth.是固定短语
答
He had no difficulty（in） working out the problem，。
常见的句式，省略了in（介词+Ving，这个应该懂），所以有所不明是很正常的
答
have difficulty in doing sth