This is a financial accounting question. I understand the calculations for period 1. However, for period 2's premium expense in (a) I did (750000/100) x (8-3) and did not get 18300. Also for (b) I do not understand how the answer is 1860 because [(840000 x 0.8) - 750000]/100 x 3 is not 1860. Can someone explain what I am doing wrong please?? The answers are given.

Ex 13-151-Premiums Irwin Music Shop gives its customers coupons redeemable for a poster plus a Bo Diddley CD One coupon is issued for each dollar of sales. On the surrender of 100 coupons and $5.00 cash the poster and CD are given to the customer. It is estimated that 80% of the coupons will be presented for redemption. Sales for the first period were $700,000, and the coupons redeemed totaled 420,000. Sales for the second period were $840,000, and the coupons redeemed totaled 750,000. Irwin Music Shop bought 20,000 posters at $2.00/poster and 20,000 CDs at $6.00/CD Instructions Prepare the following entries for the two periods, assuming all the coupons expected to be redeemed from the first period were redeemed by the end of the second period