For a standard normal distribution, if , what is the value of ?

About what percentage of the data lies within 2 standard deviations of the mean in a normal distribution?
The probability that a random variable is greater than or equal to z standard deviations from the mean in a standard normal distribution is p%. What can be said with certainty about the probability that the random variable is less than or equal to –z standard deviations from the mean?
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