One thing missing from the reddit story is how the author knows their sister is using ChatGPT to obtain the answer as opposed to validating work. I'll often punch a calculation I've already done in my head into a calculator to confirm for myself I landed on the right answer. Or come up with a solution to a problem, find myself wondering if there's a better solution or just feel like it doesn't quite look right and search for solutions online to compare and contrast. I'm not using the calculator or search to replace my work, I'm using it as the sniff test. But you couldn't determine that just by knowing my inputs to the calculator and google. You'd have to see the entire process.

Only in a few fields. You can have a successful career publishing papers in the social sciences, or justifying decisions in middle-management, without being able to know whether an effect size passes the sniff test - actually not knowing will probably help you make convenient mistakes.

24 plus 7 is 41 or 94 or 3 and 4/7 are all obviously wrong, but only if you have developed number sense. I've had students who wouldn't bat an eye at any of those solutions.