The manager of a small postal substation is trying to quantify the variation in the weekly demand for mailing tubes. She believes that this demand is Normally distributed. She knows that on average 100 tubes are purchased weekly and that 90 percent of the time, weekly demand is below 115. The manager wants to stock enough mailing tubes each week so that the probability of running out of tubes is no higher than 0.05. What is the lowest such stock level?