From a mathematician's perspective regarding the two formulas below to calculate discounted price...
e.g. Apple is normally sold for $\$100$, but today you can get them for $20\%$ cheaper!
FORMULA #1: $\quad100 - (100 * 0.2) = \$80$ is what I pay.
FORMULA #2: $\quad100 * (1 - 0.2) = \$80$ is what I pay.
Are the two options a "subjective question and matter of preference"? Or is there a logical explanation on why one is superior over the other? If it depends on the situation, I'd appreciate an example of such situation.
If I were to ask "which is a better formula?" it may be a subjective question. I'm not.
The difference between these two formulas, on a psychological level, is this: Do we calculate the size of the discount, and then subtract that discount from the total? Or rather, do we calculate the rate that we will pay after the discount, and then apply that to the total?
In the first case, it goes like this: The price would be $\$100$, but a $20\%$ discount takes off $\$20$, so now it's only $\$80$.
In the second case, it's more like: The price would be $\$100$, but with a $20\%$ discount, I only have to pay $80\%$ of that, which is to say, $\$80$.
Thus, one difference between these two formulas is that, in the first one, you're directly presented with the amount that you save, and in the second one, that part is implicit.
The only other difference that I see only arises when the price is a less "round" number. For example, if the price is $\$85$ originally, then in one case you have to calculate $(0.2)(85) = 17$, and then $85-17 = 68$, while in the second case, you're calculating $1.0-0.2 = 0.8$, and then $(0.8)(85) = 68$. If one of those sets of arithmetic tasks seems easier than the other, then you've found your preferred formula.