An Accountant! Shouldn't he know this? He is after all PAID to work with numbers. Thankfully, I don't work at the same company or I would be scared shitless for my job as I can't even imagine the accounting errors happening.
After tyring to explain it for a few minutes, I just had to lay it out like I was talking to a kid in fourth grade.
Me: OK, say you have something that is $100, and you want to increase the
price by 10%. How would you do that?
Him: Multiply it by 1.1.
Me: Right, you want to multiply the number by 1 plus an additional 10%.
So your $100 now equals $110.
Him: I increased it by 10%, so I should just be able to take 90% of that right?
Me: No, because you are now working with a higher starting number. 10% of
$100 = $10, so that is your increase, but 10% of $110 = $11. So if you decreased by that amount you will not get the original number, you would actually get a lower number $99.
Him: Oh, I get it! So how do you go from the higher number back to the original.
Me: It is a little more complicated and since it took you so long to understand the first part (about 15 minutes by this time), there is no way that I am going to go into that with you.
The whole conversation reminded me that the human race is probably doomed and reminded me of this great math quiz answer (even though it doesn't have an elephant).