Robert Swirsky — he didn’t get it wrong. 15 is the correct answer and 3x5 is not just the same, it is identical to 5x3. Mathematically, there is no difference whatsoever between them. The difference exists only because of a specific, narrow-minded approach to do the computation, but it’s not real.
This is not a matter of democracy, it’s a matter of mathematics.
Also imagine explaining to the same students two lessons later the commutative properties of mathematics and that 3x5 and 5x3 actually are the same thing. He will think the teacher is either an idiot or a pedantic asshole, and neither is good for the future teacher-student relationship.