I've just been looking up the solutions on Youtube. I'm a college student so I really don't have time to spend 3 hours on a single solution, and besides, the game has nothing more to teach me about real chemistry anyway.
And yes, the last few levels are really hard. Eventually your pipelines and reactors will be preset, and you'll have to figure out the proper way to configure the reactors themselves. You also have to create complex hydrocarbons using unknown elements.
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It's taken me just about all day to figure out a solution for 'Nothing Works' I believe it's called, towards the end of the 3rd planet, only to find out that the solution I come up with doesn't even work cause my pipes get backed up after so many cycles, somehow I have to be quicker about it.
Indie game board: http://www.gamefaqs.com/boards/293-indie-gaming
I'm on the last two levels of the 5th planet (the first one with Fusion).
As someone who spent hours and hours in Minecraft making redstone circuits fast and small, I'm almost certain that this game was made specifically for me.
I'd say it probably takes me 1-2 hours to solve something, but then I usually end up spending a couple more hours trying alternate solutions or tweaking. I generally aim to be among the pack for time but using fewer reactors/symbols than average (although for some, like 2-reactor In-Place Swap), it's just not possible due to pipeline inefficiencies.