Each year a car depreciated by 9%. Calculate the number of years after which the value of the car is 47% of it's original value.
I'm aware of the compound interest formula but I don't know if it would be any use here or how to reverse it because I'm not looking for a precise number. Should I create a number such as x and try and somehow change the compound interest formula so it's inverse?
Well, to be blunt, your original post doesn't make much sense;
what's "reverse" and "inverse"?
Once more, to find the number of periods, logs ARE REQUIRED.
Finding the "value left" after a GIVEN period is a different story: (1 + i)^n
With i = -.09 and n = 8: (1 - .09)^8 = .91^8 = .47025...