I'm in the "higher education" category, yet that puzzle only took me about 2-3min of looking at it... I got briefly red-herringed by digit sums / digital roots looking promising, so spent most of the time thinking about those, but, once I'd found counter-examples, it didn't take long to crack.

That said, the "pre-school children" bit was a big hint as to how to approach it.

The use of '=' instead of, say, '→' made the math pedant in me twitch, though ;)