application of diff

1)
http://i729.photobucket.com/albums/ww291/kaka1307/DSC00535.jpg
A man sells a wooden bar in the shape of a cuboid with base area 0.5m as shown in the figure. It is known that the price $ P of the wooden bar is
given by P=16x^2+27y.where x m and y m are the length and width of the
bases of the wooden bar respectively.
a)What is the price the wooden bar if y=0.8.
b)find x and y so that the price of the wooden bar is the least.
c)If 0.4 <= x <= 0.6,find the least price of the wooden bar.

ans:
a)27.85
b)x=3/4,y=2/3
c)28.26

pls show steps .
thx a lot