If you were keeping it a decimal it would be 6.25 with a line above 25. If you wanted it as a fraction you would just do 6 1/4 or 25/4 if you want it improper.
Assume a binomial distribution. Then the mean of the distribution is , where is the number of people taken in a sample and is the probability of the mutation occurring in any given person, and the standard deviation is . Then in this case, the standard deviation is .