A rectangular storage container with an open top is to have a volume of 60 cubic meters. The length of its base is twice the width. Material for the base costs 60 dollars per square meter. Material for the sides costs 9 dollars per square meter. Find the cost of materials for the cheapest such container.

This is what I have so far but the answer never seems to come out right.
l=2w
V=lwh=2w^2h
h=60/2w^2=30/w^2
Total Surface Area=2w^2+2(2w)(30/w^2)+2w(30/w^2)
=2w^2+(120/w)+(60/w)
=2w^2+(180/w)
Then you add in the prices of the base and sides
=(60)2w^2+(9)(180/w)

Question

A rectangular storage container with an open top is to have a volume of 60 cubic meters. The length of its base is twice the width. Material for the base costs 60 dollars per square meter. Material for the sides costs 9 dollars per square meter. Find the cost of materials for the cheapest such container.

This is what I have so far but the answer never seems to come out right. 
l=2w 
V=lwh=2w^2h 
h=60/2w^2=30/w^2 
Total Surface Area=2w^2+2(2w)(30/w^2)+2w(30/w^2) 
=2w^2+(120/w)+(60/w) 
=2w^2+(180/w) 
Then you add in the prices of the base and sides 
=(60)2w^2+(9)(180/w)

anonymous · Answer

=120w^2+(1620/w) 
Then take the derivative 
=240w-(1620/w^2) 
Set that equal to 0 and solve for w 
240w=1620/w^2 
w^3=1620/240 w
w=(cbrt(6.75) 
Then you plug that back into your equation 
=120(cbrt(6.75)^2)-(1620/(cbrt(6.75))) 
but apparently this is wrong...help please? Where did I go wrong?