How does he know this stuff?

Here's a little conversation after dinner tonight:

Logan: Mommy, what's 60 + 70?
Camden: 60 + 70 equals 130
Me: [totally stunned} how do you know that?
C: easy, 60 + 60 is 120
L: [equally stunned with a big proud smile on his face] ok, here's a tricky one: what's 60 + 78?
C: [without hesitation] 138.

How does he know this stuff? The best part was Logan's priceless smile and sheer pride that his little brother knows this stuff.