int main()
{
	int s[10],c[10],i,j,n,k;
	float sum,g[10],ave;
	i=0;
	j=0;
	k=0;
	sum=0;
	scanf("%d",&n);
	while(i<n)
	{
		scanf("%d",&c[i]);
		i++;
	}
    while(j<n)
	{
		scanf("%d",&s[j]);
        j++;
	}
	i=0;
	while(i<n)
	{
		k+=c[i];
		i++;
	}
	j=0;
	while(j<n)
	{
		if(s[j]>=90)
			g[j]=4.0;
		if(s[j]>=85&&s[j]<=89)
			g[j]=3.7;
		if(s[j]>=82&&s[j]<=84)
            g[j]=3.3;
		if(s[j]>=78&&s[j]<=81)
			g[j]=3.0;
		if(s[j]>=75&&s[j]<=77)
			g[j]=2.7;
		if(s[j]>=72&&s[j]<=74)
            g[j]=2.3;
		if(s[j]>=68&&s[j]<=71)
			g[j]=2.0;
		if(s[j]>=64&&s[j]<=67)
			g[j]=1.5;
		if(s[j]>=60&&s[j]<=63)
			g[j]=1.0;
		if(s[j]<=60)
            g[j]=0;
		j++;
	}
	j=0;
	i=0;
	while(j<n)
	{
        sum+=g[j]*c[i];
		i++;
		j++;
	}
	ave=sum/k;
	printf("%.2f",ave);
    return 0;
}