How do production points work in Civilization 5?

What is the math behind tile production vs. production used to produce a unit? For example, let's say I have a city whose worked tiles (including its own) show 4 hammers.

I then queue production of a unit which is listed as costing 75 production points.

How long does it take? Are there any "wasted hammers"?

Solution 1:

Each turn you gain 4 production points from your 4 hammers. So assuming everything stays the same, it would take 19 turns to complete your production. That would produce 76 points, so the extra point is saved and applied to your next production choice, so you'd get 5 points on its first turn.