What is the net present value of periodic payments of 10, 20 and 30 currency units with a discount rate of 8.75%. At time zero the costs were paid as 40 currency units.
在贴现率为 8.75% 且定期支付额为 10、20 和 30 个货币单位的情况下，此项投资的净现值是多少？时间为零时所支付的成本为 -40 个货币单位。