Ace Leasing acquires equipment and leases it to customers under long-term sales-type leases. Ace earns interest under these arrangements at a 5% annual rate. Ace leased a machine it purchased for $680,000 under an arrangement that specified annual payments beginning at the commencement of the lease for four years. The lessee had the option to purchase the machine at the end of the lease term for $180,000 when it was expected to have a residual value of $240,000. (FV of $1, PV of $1, FVA of $1, PVA of $1, FVAD of $1 and PVAD of $1) (Use appropriate factor(s) from the tables provided.)

please show me how did you found purchase option