int main(){
    int num_classes;
    cin>>num_classes;
    int pts[10];
    for(int i = 0; i < num_classes; i++){
        cin>>pts[i];
    }
    int total_score = 0;
    int total_credits = 0;
    int tmp;
    for(int i = 0; i < num_classes; i++){
        cin>>tmp;
        total_credits += pts[i];
        if(tmp >= 90)
            total_score += 40 * pts[i];
        else if(tmp >= 85)
            total_score += 37 * pts[i];
        else if(tmp >= 82)
            total_score += 33 * pts[i];
        else if(tmp >= 78)
            total_score += 30 * pts[i];
        else if(tmp >= 75)
            total_score += 27 * pts[i];
        else if(tmp >= 72)
            total_score += 23 * pts[i];
        else if(tmp >= 68)
            total_score += 20 * pts[i];
        else if(tmp >= 64)
            total_score += 15 * pts[i];
        else if(tmp >= 60)
            total_score += 10 * pts[i];
    }
    printf("%.2f",(float)total_score / (float)(total_credits * 10));
}