A car deceased in value by 25% in the first year. purchased for $28,000, what is the car worth today.

Try this: Principal times (1+interest rate as a decimal fraction) ^ (number of yrs)

In this case, Principal = $28,000; interest rate = - 0.25; # of years = 1

So Decrease in value = $28,000 (0.75) ^1 = $21,000.