Question

An automobile depreciates by 20% per year. how soon will it be worth only half its original value? [hint: depreciation is like interest but at a negative rate r.]

Answer 1

1/2p=p (1-0.2)^t
1/2=(1-0.2)^t
1/2=(0.8)^t
Log (1/2)=t×log (0.8)
T=log(1÷2)÷log(0.8)
T=3.1 years