since the price is decreasing by 7.5% each year ( that is 0.075) the function that models this situation
will be as follows 70 - 70(0.075) = 70(1-0.075) = 70(0.925) for each year and for n years will be 70(0.925)^n
For 4 years later the calculator value will be 70(0.925)^4 = 70(0.7320) = 51.246 approx
we can consider this as a geometric progression having a rate of progress = 0.925
if consider the todays price of $70 as the a10 term of the progress (cause we must find the price of the calculator for ten years ago) then the first term would be a1. so the 10nth term a10 would be
a1(0.925)^10 = a10 = 70 -->>> a1 = 70/0.925^10 = 70/0.45858 = 152.645 so its value 10 years ago
it was $152.645