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4. From past experience, it is known 90% of one-year-old children can distinguish their mother's voice from the voice of a similar sounding female. A random sample of 20 one-year-olds is given this voice recognition test. [8 marks] (Round your answers to three decimal places if necessary.) a) Find the probability at least 3 children do not recognize their mother's voice. b) Find the probability all 20 children recognize their mother's voice. c) Let the random variable x denote the number of children who do not recognize their mother's voice. Find the variance of x. d) Find the probability that at most 4 children do not recognize their mother's voice