We shouldn't necessarily regard 5 as the correct output. Sure, almost all of us choose to make division higher precedence than addition, but there's no reason that has to be the case. I think a truly intelligent system would reply with 5 (which follows the usual convention, and would therefore mimic the standard human response), but immediately ask if perhaps you had intended a different order of operations (or even other meanings for the symbols), and suggest other possibilities and mention the fact that your question could be considered not well-defined...which is basically what it did.