Example 5: Current AA+ corporate bond yields in the market are 8% per annum. An existing AA+ corporate bond with a face value of $100 000, paying 10% per annum halfyearly coupons, maturing 31 December 2020, would be sold on 20 May 2015 at what price? I'm already know the answer, but dot know why the k=141/180? why the first period is count from 1 Jan 2015 ? why 1th Jan is the last coupon day?