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I could swear this one was an episode of Outer Limits. The members of the Carmichael family are all overweight, and would like to thin down a bit, so Mr Carmichael splurges on the newest model robot cook. The roboservitor can be programmed for any kind of diet they want. Twenty pounds in three months? Put the numbers in, it'll figure out the meals to reach the goal. It may mean toast and coffee for breakfast, plain lettuce and tomato salad for lunch with a hard-boiled egg, and steak with peas and coffee for dinner. And there's no telling what it might do if you try to cheat.
On 27 January, 1973, the story was broadcast on the revived X Minus One radio show. Tapes or CDs might be available if you search.
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