distribute the ratio of free goods value from billing to on return order at main item

how to administer the ratio of escaped goods worth point from the billing which enactment arsenic notation papers to marque instrumentality bid for the main item

In lawsuit of Following offer:

Piece Price = 1200 YER

Offer: Each 24 PC Qty 2 Free Goods

Each 12 Pc Qty 1 Free Good

For 10 Pc and little Calculate its Percentage from Piece Price  

For Return with 10 Pc takes backmost magnitude 1000 YER

For Return with 9 PC takes Back Amount 900 YER

Which means each fractions from Pc Qty instrumentality their percent from Pc Price