1. The problem statement, all variables and given/known data This was an example but I am not sure why there is a difference in the way they are calculating the standard deviation. A type of battery is produced with an average voltage of 60 with a standard deviation of 4 volts. A: If 9 batteries are selected at random, what is the probability that the total voltage of the 9 batteries is greater than 530? B: What is the probability that the average voltage of the 9 batteries is less than 62? 2. Relevant equations 3. The attempt at a solution Problem A: For the stdev they just: sqrt( 9 * 4^2) = 12 Problem B: For the stdev they just: 4/sqrt(9) = 1.3333 I get why Problem B is the way it is. We are dealing with a sample and we have the population stdev and we need to approximate the sample standard deviation by dividing by sqrt(n). But for Problem A, I feel like I am missing something simple. Any clarification would be great.