int main(){
	int n,q=0,i,xuefen[10],chengji[10];
	double p=0,zong,GPA[10],xuefenGPA[10];
	scanf("%d",&n);
	for(i=0;i<n;i++){
		scanf("%d",&(xuefen[i]));
	}
    for(i=0;i<n;i++){
        scanf("%d",&(chengji[i]));
	}
    for(i=0;i<n;i++){
		if(chengji[i]>=90){
		GPA[i]=4.0;
		}else if(chengji[i]>=85&&chengji[i]<90){
		GPA[i]=3.7;
		}else if(chengji[i]>=82&&chengji[i]<85){
		GPA[i]=3.3;
		}else if(chengji[i]>=78&&chengji[i]<82){
		GPA[i]=3.0;
		}else if(chengji[i]>=75&&chengji[i]<78){
		GPA[i]=2.7;
		}else if(chengji[i]>=72&&chengji[i]<75){
		GPA[i]=2.3;
		}else if(chengji[i]>=68&&chengji[i]<72){
		GPA[i]=2.0;
		}else if(chengji[i]>=64&&chengji[i]<68){
		GPA[i]=1.5;
		}else if(chengji[i]>=60&&chengji[i]<64){
		GPA[i]=1.0;
		}else{
            GPA[i]=0;
		}
	}
    for(i=0;i<n;i++){
		xuefenGPA[i]=GPA[i]*xuefen[i];
	}
    for(i=0;i<n;i++){
		p+=xuefenGPA[i];
	}
    for(i=0;i<n;i++){
		q+=xuefen[i];
	}
	zong=p/q;
	printf("%.2lf",zong);

	return 0;


}
