So , if we can break the MD5 hash we will earn huge money from this kind of websites, do you have any solution to do this?
I am not really interested in this, but I can tell you that there are many attacks that can be done on MD5 depending on how this is implemented and what the exact challenge is.
Other then that, you can always brute force it. You will probably be able to hash billions of values per second with MD5 using your graphics card.