Please do not use "and/or" in either formal or informal writing.
In common English, the "or" is a "non-exclusive or" which means "either A or B, or A and B". When I say "I can have a banana OR I can have coffee" then I am also OK with having both. Having a banana does not prevent me from having coffee.
The "exclusive or" which means "A or B, but not both" properly is used in legal documents - where the text must be completely unambiguous - and in logic circuits, where logic gates differentiate between "or" and "exclusive or" which is often denoted as "EOR" or "XOR" to distinguish it from "OR".
Unfortunately, the use of "and/or" in English writing has proliferated in recent decades. I attribute this trend to a growing laziness in written communications - not taking the time to structure sentences to be clear, but just tossing in an "and/or" to cover the lapse.
But a basic rule of good writing is to not write something that you would find awkward if speaking aloud. And I don't know anybody who says "I'm going to buy some food at a grocery store 'and/or' a bakery"!