Percent Increase between old and new values - kindof...

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I'd like to check if a formula I have been using is wrong.

I am studying and came across the following in how to calculate the percent increase/decrease of a value:

% increase = Increase ÷ Original Number × 100

For example if I buy a product for 2 and sell it for 4, the formula is:

(4-2)/2 x100 = 100% increase.

I get all of this. My issue is that I was using another formula until I came across this one (which is obviously wrong in this context) and I am amazed I have had it wrong for so many years; I feel as if it is a valid formula which I was taught a long time ago and maybe I am just applying it wrong, it is:

new value /orginal value x100

So in this example we would have:

4/2x100 = 200%

My question is, in what context do you use this forumla (if at all)?

Sorry for the somewhat vague question, it's just I can't remember how I aquired this formula; my memory is usually very good, which is why I feel as if it is used in some context and I have just mixed it up with something else.

Thanks,