The empirical rule says that approximately 68% of a normal distribution falls within one standard deviation of the mean; 95% within two standard deviations; and 99.7% within three standard deviations.
The interval (500, 700) is half of the portion of the distribution that falls within two standard deviations of the mean (the whole portion being the interval (300, 700)). So, the desired probability is [tex]0.5\times0.95=0.475[/tex].
