I’m sure that there are some great ways of memorising formulae, but you won’t need them here.
If you are using these formulae as an example, so you can learn a method for memorising, try reading this thread and ignore everything below this.
Apologies if what I’m about to write seems condescending but I’d rather over-explain than under-explain.
The first point is 6, 7, 8 and 9 are the same equation. Think of a loss as a negative gain and it’s clear that 6 is the same as 7, and 8 is the same as 9. To see that 6 and 8 are the same, all you need to do is rearrange. Start with equation 6, multiply both sides through by 100, then divide both sides by 100 + Gain%.
Incidentally, equation 5 is the same as what I assume is equation 4, by the same logic of losses being negative gains.
All you need to do from there is know what equations 4 and 6 mean, and they seem self explanatory
Divide the gain by the cost price to get the proportional gain, multiply by 100 to make that a percentage. You use the cost price because you want to know how much better off you are than you were before, and you obviously paid for it before you sold it.
You know the percentage gain and the cost but not the selling price (incidentally, why you’d ever know what proportion of money you gained and not the selling price, which is required to calculate that proportion, I’ll never know). For this you just rearrange the gain% equation, bearing in mind the Gain is SP - CP. (Multiply through by CP, divide by 100, add CP, simplify)