5 The weights of letters posted by a certain business are normally distributed with mean 20 g . It is found that the weights of \(94 \%\) of the letters are within 12 g of the mean.
- Find the standard deviation of the weights of the letters.
- Find the probability that a randomly chosen letter weighs more than 13 g .
- Find the probability that at least 2 of a random sample of 7 letters have weights which are more than 12 g above the mean.