Learning curve rate BPP

Hey All,

Just wondering if anyone can help with this question, I am currently doing questions from the BPP P&R KIT I’m looking at question 40.8.

For those whom don’t have access to this p&r kit, the question goes like..

A company has estimated that an 85% learning curve rate will apply to the production of a new item for which the time to produce the first unit was 800 hrs.
What is the expected time to produce the 9th unit.

A. 370.86 hrs
B. 417.61 hrs
C. 477.92 hrs
D. 491.30 hrs

I got as far as.. log (0.85)÷log (2)= -0.2344652

Then I got confused, looking back at the answer doesn’t help as I feel the way the answers are written doesn’t necessarily correlate to how I’ve been taught.

I was thinking 800hrs X 9 units (-0.2344652)

Please help!