12 eggs in a box; or, how come I am mathematically challenged?

Friday night, Cloud and I contemplated the weekend's breakfasts. Last weekend, we had purchased a box of 12 eggs to save faffing about getting one on Saturday and one on Sunday to provide 4 eggs between us for each day. Somehow though, I was convinced this left us with 6 eggs and that if we got another box of 6 this would leave us with way too many eggs than we would need...

It took some great persuasion and thinking on my part to recalculate the problem. I am deeply ashamed. We had watched Blackadder the previous night with the "2 beans plus 2 beans = some beans" sketch. The whole experience left me feeling like I had just managed to say that "12 egss minus 4 eggs, minus another 4 eggs leaves 6 eggs!" Thus making me the very dim kid sister of a boy Cloud and I once passed in town:

Mum! Three and another three and another three is NINE!

He was about 6 years old...