n=int(input())
m=list(map(int,input().split()))
l=list(map(int,input().split()))
a=0
def Sort(x):
if x>=90:
return 4.0
elif x>=85:
return 3.7
elif x>=82:
return 3.3
elif x>=78:
return 3.0
elif x>=75:
return 2.7
elif x>=72:
return 2.3
elif x>=68:
return 2.0
elif x>=64:
return 1.5
elif x>=60:
return 1.0
else:
return 0
for i in range(n):
a=Sort(l[i])*m[i]+a
b=sum(m)
c=a/b
print(‘%.2f’% c)