algebra
posted by talisa .
Susie purchased a laptop computer for $2000 at the beginning of the year in 2008,the value of the computer decreased by 20% each year. Susie wants to purchase a new laptop computer when the value of hers falls below $500. When will Susie be able to purchase a new laptop?

so you are solving
2000(.8)^n = 500
.8^n = 500/2000 = .25
take log of both sides
log .8^n = log .25
n log .8 = log .25
n = log .25/log .8 = appr 6.2 years