Three is a Magic Number (or is it four?)

I can't do any math in my head anymore. Even simple stuff. I noticed it for the first time when I was in Kroger when they had half-gallons of milk on sale, 8 for $10, and I was trying to figure out if it would be cheaper to buy a gallon or two half-gallons. It went something like this:

"Eight goes into 10...uh...wouldn't that be 10 goes into 8? No, I was right the first time. It's one, then you have to subtract...uh..but then multiply it by two...how much is a gallon? Goddammit, where's my calculator?"

It has even affected me at work. I was trying to sound all authoritative to our director the other day: "The Company will have to pay a 10% excise tax on any amounts refunded after March 15. For example, if we end up processing $500,000 in refunds, we will have to pay...uh...um..." I actually had to ask, "What's 10% of $500,000?" Completely blew my authoritative facade. And probably my merit raise, as well. Not that I'd know the difference.

Me: "This raise seems kind of low. I thought we discussed a higher figure."

My Boss: "Oh no, it's right. Um, in fact, it's MORE than what we agreed to. See, you just have to divide this annual total by...um...8 for a monthly total, since, um, this raise is effective until April of next year. Once you multiply that number by 12, then you will see that you are actually earning MORE than what we agreed to.. Yeah. Heh heh."

Me: "Huh? Oh. Okay...great! Thanks!"

So if we ever go out to lunch together and have to split the tab, you will notice that I just sit there until someone tells me how much to put in. If it's one of those things where the tab goes around the table for each person to calculate their share, you'll notice that I sit there for a minute looking pensive, then take out some bills and say, "Is this fair?" I'm really asking. I'm not trying to short-change anyone. Just help me out.
Or if you need a ten-spot, just add it on. I won't notice.