CFA 2012 AM Mock: Q7 Derivatives move, 2012, following, example, method

bchadwick

In the AM derivatives case they note the following:
Up Move on Stock 15%
Down Move on Stock 10%
I was under the impression that to calculate the down move you took the Up move 1.15 and take the reciprocal 1/1.15 = .8696
Therefore U = 1.15 & D = 0.8696
They are using U = 1.15 and D = 0.9 which is not the reciprocal. For every EOC and example in the text they used the reciprocal method…any ideas? These should be easy points for me and now I’m shakey on the easiest part…ugh!!