FRmini exercises - question 1

A butcher sells $300,000 of meat at a consistent mark up of 25%. His inventory at the start of the year was $15,000. This had increased by 20% by the end of the year. Calculate the purchases for the year. i did : sales 300,000 less cost of sales opening inventory 15000 purchases x less c/inventory (18000) ----------------- (225,000) ------------ Gross profit (markup 25%) 75,000 and finally i'm getting purchases amounted to : $ 228000 why is my answer wrong ?