[SOLVED] Optmization - Cost

**Question: A rectangular storage container is to have a volume of 10 meters cubed. The length of the base is twice the width. Material for the base costs $10 per square meter. Material for the sides and top costs $6 per square meter. Find the cost of the materials for the cheapest such container. ANS: $191.28**

So I have $\displaystyle A = 40w^2 + 36wh $ and $\displaystyle V = 2w^2h$

Then rearranging V to get $\displaystyle h = 10/2w^2$

What am I doing incorrectly? I subbed h into the A equation and solved dA/Dw equal to zero. . . $\displaystyle 0 = 80w - 180w/w^2$

I got w = 1.5 and h = 20/9