Evie scored a 90 on her calculus final, which is one standard deviation above the class mean of 80. The final is broken into two equally weighted parts A and B so that the sum of the scores on both parts is the final score. If her score on Part A had a z-score of 1 and her score on Part B a z-score of 1/2, what is the standard deviation of the class's scores on Part B, assuming both parts to be normally distributed?