Suppose you own a fleet of vans which cost you per van:

Initial cost of $30k; Annual cost of $12k in the 1st year, $15k in the 2nd, $20 in the 3rd, $25 in the 4th; the salvage value is $20k at the end of 2nd year, $17k at the 3rd, $12K at the 4th.

How often should you replace vans if the discount rate was 5%?

Please show work in excel spreadsheet and thank you!