I wrote a letter in reply to his questions.

我写了封信答复他的疑问。

youdao

We wrote a letter in reply, refuting his erroneous views.

我们回了一封信，驳斥了他的错误观点。

youdao

The author whom you criticized in your view has written a letter in reply.

你在评论中批评的那个作者已写了一封回信。

youdao

The two firms submitted an application to the FDA way back in June 2000, but a July 2001 letter sent in reply asked for more data.

FORBES: Novartis Pushes Ahead On Pipeline Snags

In his letter of reply, the PM said that "after a quarter of a century on the front bench I entirely understand your desire to pursue other interests".

BBC: Lord Strathclyde resigns from cabinet