You probably do not have a child of 7 years old because they do not know at that age what is a prime number.

Second, basic math still that you never or rarely use or with very large time between usage might get rusty. You may understand the concept but not find the optimal solution. The way you are responding here shows quite a lot about how you are short sighted by instant-failing someone with a single question instead of trying to asses the whole person as much as you can. On you side, you are wasting opportunity to have a great person that could be a key player in your team by bringing other set of skill on the table.

You're not testing for "basic math skills" here. What you're testing for is more like "immediately retrieves an irrelevant math fact after many years of having no need to think about it."

Look, if you think this sort of thing allows you to identify great candidates, good for you. But in my experience, not only is this kind of practice stupid on its face, but it leads to engineering orgs packed with people who are good at memorizing trivia but terrible at solving real problems.

I think the key problem here is that is is a bad programming question. If you know anything about prime numbers then coming up with an answer is trivial. If you expect a more optimized solution, then you are really only gauging the interviewee’s understanding of prime numbers. So effectively the interview is more about mathematics than it is about programming or problem solving.

You happen to remember a particular piece of knowledge, so you project that expectation onto others. Theory of mind.

> yes, we expect professional software developers to have basic maths skills

Skill != knowledge. "What is a prime number" can be looked up and understood by any competent programmer in <5 minutes.

> "what is a prime number" is taught to 7 year olds, it's not vector calculus

Then it's reasonable to expect that an interviewee would be able to learn it as well, given the same resources. It does not however follow that an interviewee would inherently have that knowledge, just because 7 year olds are taught it.

Bottom line is, you're making too many assumptions about complete strangers.