求英语高手〜这句话对不对I am writing to thank you for your help,which increased 20% sales last month

求英语高手〜这句话对不对
I am writing to thank you for your help,which increased 20% sales last month

不对.错句.
i am writing to you for your help,whith which we increased 20% sales last month.

我写这封信是为了感谢你的帮助,上个月销售额增加了20%

我写信是想感谢你的帮助，上个月销量提升了20%

有问题。
I am writing to thank you for your help后面应直接讲要帮助什么。which代表什么？意思不明。

I am writing to thank you for making our sales increase 20% last month.不知道这句行不行。你那句which指代不明