TDemand for stereo headphones and music players for joggers has caused Nina Industries to grow almost $50$ percent over the past year. The number of joggers continues to expand, so Nina expects demand for headsets to also expand, because, as yet, no safety laws have been passed to prevent joggers from wearing them. Demand for the players for this year was as follows:

o be reasonably confident of meeting demand, Nina decides to use $2$ standard errors of estimate for safety. How many additional units should be held to meet this level of confidence?