A manager at Saks Fifth Avenue wants to estimate the number of customers to expect on the last shopping day before Christmas. She collects data from three previous years, and determines that the crowds follow the same general pattern. When the store opens at am, people enter, and the total number in the store doubles every minutes. When the number of people in the store reaches security guards need to be stationed at the entrances to control the crowds. At what time should the guards be commissioned?