go to the locations info-screen. you can see the the production of food an water there. normally it shows maddows. i assume you have a rat trap. left-click on the maggots, you'll see rats then. a right-click brings you back to maggots. the numer right shows how many DR of rats will be produced. lets say 3DR. your gang member use up 1DR, so you have a net production of 2DR. a single rat provides 5DR of food, so your gang member produces 2 rats in 10 days. the collects these in a random building (that must not be the one containing the trap). you can normally pick them up from there.
the better types of food can not be produced in every location. just try it out in the info-screen. if you can't select snakes here, they can't be produced.
you have to select the apropriate type of food for your tool. if you leave a rat trap at a location, but select maggots in the info-screen, the member will produce maggots and leave the trap unused.
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