I need to calculate the pay back period for a company which has the following information:-

Initial investment 200,000

year 1 - 30,000
year 2 - 50,000
year 3 - 90,000
year 4 - 120,000

in year 4 there is a estimated residual value of assets of 40,000

what do I do with the residual value? Thankx.