I got the same result as you, but in my mind the expression "twice as likely to purchase" could just as easily have meant P(C) = 2 * P(D) / 1 + P(D) (yielding 0

I got the same result as you, but in my mind the expression "twice as likely to purchase" could just as easily have meant P(C) = 2 * P(D) / 1 + P(D) (yielding 0.35824) rather than 2 * P(C) (yielding 0.32841).