Math/Environmental/Chemistry

Read in its entirity before you post a price.  It's not a difficult question im asking, just need to understand why my answer is different from another tutor.

What I need to know is why the correct Volume math for this question is not  D*W*L/27 to my cubic yards...my answer is 183.33, however a tutors answer is 177.88  Which one is right?  If it is the tutor please explaine why.    Do not need the whole answer....my math is done...just need to understand why his is different.

Question follows:

You are the project manager of an environmental company that was hired to cleanup a historical arsenic contamination site. Soil sampling results indicated the area impacted with arsenic above action levels is 30 feet by 55 feet. The depth of the arsenic plume is three feet below ground surface. The land owner wanted to clean this site for future development. Answer the questions below to help you prepare a cost estimate for this portion of the project. Your solutions and any assumptions to justify your estimate must be shown.