How did he do that?

Lately Robert has taken to quizzing us on math. He likes to ask us what 10+5 is and what 5+10 is. He doesn't really understand, but he is getting a sense that math is something we do (even mom who tries to avoid math as much as possible). So he was doing his usual quizzing tonight and I whispered to him to ask what 3x5 is. Jamie answered right away, whether it was because he actually knew this, or because the last two questions had an answer of 15 I am unsure. Then I whispered to Robert to ask what 30 divided by 2 was. Jamie thought for a few moments then answered 15. "How did you know figure that out?" I asked, thinking that maybe it was just because the last answers were 15. He tells me that 10+10+10=30, so you take the one 10 and split it in half which is two fives and add them to the two tens and you get 2 fifteens. So the answer is 15. How's that for problem solving?