If someone really can't do that, then either they don't know how to count (very unlikely), they didn't understand the question, or they are really bad at transl

If someone really can't do that, then either they don't know how to count (very unlikely), they didn't understand the question, or they are really bad at translating what they would do on paper into a program. All disqualifying. Another possibility is nerves. Early in my career I was very nervous in interviews and I have sympathy. I have fucked up the most basic search algorithms completely and spewed gibberish when asked to explain the complexity of my solution. I have sympathy for that, but the only way to get through interview nerves is exposure therapy. At least in my experience.