A exponential function would represent it nicely f(x)=b*a^x Reason being a decrease of % a = 1+r r = -25% = -0.25 And b being what the original cost of the book f(x)=85*0.75^x Where f(x) is the price after x amount of previous owners
Since the value decreases by 25% each time, the next cost is the current cost times 0.75 (since new cost is 75% of current cost). To find the value of the book after x times sold, you can use: Just plug in the number of times it is resold to find the value.