题目内容：

the population of snowtown was 4500 in 1990.In 1994,it had decreased to 3600.Find the percent decrease in population during those four years.如果有公式也一起给我,给个详解

优质解答

原意：1990年……的人口是4500人,在1994年则下降到3600.求下降率.
（4500－3600）/4500＝20％