David Niven can purchase a new laptop computer today for $2500. The laptop provides an economic benifit of $900 per annum and has an expected life of 6 years. Laptops are expected to decline in price by 10% per annum and Davids discount rate is 10%. When is the best time for david to purcahse the laptop.?

Im stuck on this question...

I tried as shown in an example but i dont think it is correct...

some help would be great..

heres what i have so far:

Year : Cost : PV of Savings : NPV at purchase : NPV today

0 : -2500 : +900 : -1600 : -1600

1 : -2250 : +900 : -1350 : -1227.27

2 : -2025 : +900 : -1125 : -929.75

3 : -1822.50 : +900 : -922.25 : -692.90

4 : -1640.25 : +900 : -740.50 : -505.77

5 : -1475.975 : +900 : -575.975 : -357.64

They way i calculated NPV today was....

-1350/1.1^1

-1125/1.1^2.....so on..

am i correct?? where have i gone wrong if im wrong?

THANKS