The following data were obtained from a survey of college students. The variable X represents the number of non-assigned books read during the past six months.
x 0 1 2 3 4 5 6
P (X=x) 0.55 0.15 0.10 0.10 0.04 0.03 0.03
What is the standard deviation of X? Place your answer, rounded to two decimal places in the blank. For example, 4.56 would be a legitimate entry.