Forest rangers at a state park have observed a decline in the rate of trees per acre over a 5 year period. At the beginning of their observations, there was an average of 48 trees per acre. At the end of the 5 year period, there was an average of 42 trees per acre. If this trend continues how many trees per acre will there be in 4 1/2 years from the end of the 5 hear observation period?