when studying FX gain and losses we have sometimes to calculate the interest expense of a purchase based on average rate and not the spots rate. How can I calculate that? Here are the spot rates I have:

Jan 10, Year 1: $1.28

March 17, Year 1: $1.29

May 1, Year 1: $1.31

June 30, Year 1: $1.34

July 31, Year 1: $1.37

*Exchange rates changed evenly between the dates indicated above.