Optimization of cake pan volume from area of pan It was difficult to accurately word this question, so hopefully a bit of context will clear that up. 
Context:
I have a cake dish that is made by cutting out squares from the corners of a 25cm by 40 cm rectangle of tin. 
                          40cm

                 _____________________
                |_|                 |_|
       25cm     |                     |
                |                     |
                |_                   _|
                | |_________________|_|

A 3D cake tin is made by folding the edges once the squares have been cut away. 
What size squares must be cut out to produce a cake dish of maximum volume? 
My working: 


*

*I know that the area of the pan without the squares will be:
(40 - 2X) * (25-2Y) = Volume

*But that's about all I can wrap my head around. I know that the pieces cut off are sqaures, so they will have the same width and length. But that's all I can think of doing... 
How exactly can I find what size square will produce a maximum volume for the container? 
I'm pretty terrible at math, and I know it looks like I've done nothing to try and solve this. But I honestly am at a bit of a loss. 

Hint: Suppose we remove $x\times x$ squares. The "finished" pan will have length $40-2x$, width $25-2x$, and depth $x$. So its volume $V(x)$ is given by 
$$V(x)=(40-2x)(25-2x)(x).$$
You want to choose $x$ that maximizes $V(x)$. Note that we will need $0\lt x$ and $2x\lt 25$. 
