I am having headaches whenever question requires choosing base appropriately when manipulating percentage related problems. I am sure I haven't made any sense so far, so lemme choose a example problem first :
A number is increased by 20% and then again increased by 20%. By what percent should the increased number be decreased so as to get back the original number?
My init solution was like :
let there be number $x$ which is increased sequentially twice by $ 20$%$ $ . So the difference between increased number and init number $x$ would be : $ 120$%$ 120$%$ x - x $
Now what to choose as base (increased number or init number $x$ ?) to make the ratio (part to whole) and then convert it in to percent ?
This was just an example of problem I often face , so I'd welcome any concepts/analogy which will make whole base selection procedure easy . Thanks