The average length of sentences in a business letter is thirty to forty words?

The statement is false. A sentence in a business letter is as long as it needs to be, but absolutely no longer than that! In a business letter, you need to convey all the information that is required to accomplish the purpose of the letter, so sentences length is not a factor (unless it is a run on sentence, then it needs to be edited). Examples of appropriate sentences:

• At the time that I purchase this product, it was in an unopened carton with all seals intact and no damage or signs of mishandling or water exposure apparent on its exterior. (32 Words, longer than average)
• Mr. Swanson is also appropriate for this responsibility due to his many years of experience.
• It will be my pleasure to attend this meeting. Thank you for inviting me.

The length of the sentence, average or not, is not important; the information stated clearly and concisely is important.