the maths problem isn't very easy and it is difficulta to finishb to finish it

问题描述:

the maths problem isn't very easy and it is difficult
a to finish
b to finish it

选a finish的宾语就是前面的主语it, 所以不需要加it

关于主语the maths problem isn't very easy and it is difficult to finish.
关于形式主语
it is difficult to finish it.

a.
因为后面那句话是 it is difficult 已经有了主词 (it)

The maths problem isn't very easy and it is difficult to finish it.
解析:it is difficult _____中it为形式主语,真正的主语是不定式to finish it. 即:To finish it is difficult.