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.
