学生类信息，该怎么修改

class Student{
private String id;
private String name;
private String sex;
private String phone;
private String chengji;

public Student(String i,String n,String s,String p,String c){
this.id=i;
this.name=n;
this.sex=s;
this.phone=p;
this.chengji=c;

}


public String getId(){
return this.id;
}

public String getName(){
return this.name;
}

public String getSex(){
return this.sex;
}

public String getPhone(){
return this.phone;
}

public String getChengji(){
return this.chengji;
}
;
}

class Test{
public static void main(String[] args){
Student s1=new Student("学号: 017","姓名：张大华","性别：男","电话：5887185","java成绩: 60");
Student s2=new Student("学号: 018","姓名：张小华","性别：男","电话：5887185","java成绩: 80");
System.out.println(s1.getId());
System.out.println(s1.getName());
System.out.println(s1.getSex());
System.out.println(s1.getPhone());
System.out.println(s1.getChengji());
System.out.println();
System.out.println(s2.getId());
System.out.println(s2.getName());
System.out.println(s2.getSex());
System.out.println(s2.getPhone());
System.out.println(s2.getChengji());
System.out.println();
float ave;
ave=(85+80)/2;
System.out.println("两人的平均成绩为："+ave);
}
}
为什么运行结果中ave=82.0,改了分数后，结果仍为８２.０，错在哪儿？另外此类中求平均成绩可以要哪种方法？求平均成绩应该怎么求？，望各位帮忙解答，谢谢