this is how i reckon as lesson of maths with luffy would go (one piece incase your uncultured)

Zoro: If you had 10 pieces of cake

luffy: meat

zoro: - _ - fine 10 pieces of meat and someone took 2 pieces away from you how much would have

luffy: 12

zoro: no they took 2 pieces away...

luffy: no....i'd kill them and take 10 pieces from them!! then i have 12!

zoro: ....no...luffy...*face palms and gives up*