For all that, retrochic, pointing out how rubbish someone may be doesn't actually help the matter. Most people I've met who aren't mathematically minded are already aware of that, and don't need anyone pointing it out. Then the only question is if they care or not. Sadly, rather a lot of people don't seem to care. That's not really the case here anyway, it seems to me, so that we're left with someone who's being unfairly criticised for asking a question. And of course for that matter it's worth pointing out that phleb evidently knew that the "answer" Excel had given was wrong, and why was that, which in itself is worth commending. Far too many people trust what the computer says -- at least here that hasn't happened.
If people are bad at maths and don't care, criticising them for it doesn't achieve anything. If people are bad at maths but do care, criticising is just unfair. Evidently people's brains are wired differently so that what's easy to some is hard to others.
During my time at high school I was working with someone, one of the nicest kids, seriously enthusiastic and clearly very passionate about trying to learn about maths. And yet he couldn't seem even to work out how to factorise 70 as 2*5*7. That's not exactly a difficult thing to do. Poor kid couldn't get his head around it. Pointing out how bad at maths he was would be nothing other than mean. And it's this sort of attitude that I'm criticising, and I make no apologies for that. It's an attitude that helps no-one.