Growth and Decay: rational exponents?

Question

Business The value of a computer depreciates (decreases) at the rate of 25% per year. If a computer is now worth $2400, find its approximate value:

a.3 years and 6 months from now
b.20 months ago

Answer 1

at 25% per year, the value of the computer is 2400*(1-.25)^t where t is in years from now, positive in the future, negative in the past.

so the answer to a is
2400* [.75^3.5] = $876

b. 20 months ago means that t= -20/12, so the answer is
2400*(.75^-20/12) = $3876