it's not simple to explain, you need certain level of detail to understand it, otherwise what's the point. and your understanding per your post is actually not correct, every block always have the static # of bitcoins(until halved) in them, for example, there will never be a block when unlocked has no bitcoins. It's also not equations you are trying to solve, it's just unintelligent trial and error until a hash fits and everyone else verify that hash fits too. The verification has O(1) performance alas hard to find easy to verify.
How to explain Bitcoin to a newbie? It's a good exercise. This is one attempt: All transactions are posted publicly and are verified by many different computers. They receive Bitcoins for this service. Only 21 million coins will every be "minted" or "mined". Your "public key" is your account # and you can have as many account #'s as you like.