This is because when you divide 12/100 and simplify. All you need to focus on is that there are 100 tiles and twelve of them are with the letter E the simplify from there. all the other information is just there to distract you.
The empirical rule says that approximately 68% of any normal distributed data set lies within one standard deviation of the mean.
The same rule states that about 95% lies within two standard deviations of the mean. Using this rule, and the fact that any normal distribution is symmetric about its mean, you have