6.35 is 2 standard deviations below the mean of 7.13, so the empirical rule tells you 2.5% are in the lower tail of the distribution.
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The empirical rule says 95% are within 2 standard deviations of the mean, so 5% are outside that range. Those 5% are evenly split between the upper and lower tails of the distribution, so 5%/2 = 2.5% are in the lower tail, below -2 standard deviations.
The value 6.35 is 0.78 = 2×0.39 less than the mean of 7.13.
