There's a glitch that allows you to quickly click "promote" 3 times and receive all of the promotions. You only need the first promotion researched (which is why it's a glitch) and only enough money to cover the initial promotion (though the rest of the promotions will put you negative until you work off the debt). It makes the game super easy if you do this to your workers early on. Just research the first promotion and use the glitch to get one worker fully upgraded at a time and soon enough you'll be raking in the dough faster than you can spend it!
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