Most of the answers don't hit the real point of the question: the hashing transformations are non linear, and as such are very difficult (but not impossible, given enough computational power and time) to reverse.
Think about the relative difficulty of squaring a number and getting the square root. Add to that that you just have partial information and all the missing bits are important to yield the correct answer (not as in the example of cropping a number).
If after all you're still not sure, then try by yourself to reverse the steps of MD5 or any other cryptographic hash function ;-)
