Mini Exercise Questions - Cost of sales Q2

I'm having trouble calculating the opening & closing stock on Mini Exercise Questions - Cost of sales Q2 Sales of $450k with gross margin of 15% Gross profit (450 x 15%) $67.5 Cost of sales $382.5 Purchases $400k Closing stock 30% higher than opening stock? I'm having trouble working out the opening & closing stock? From the book the answers are: opening stock =$58.33 Closing stock = $75.83 Thanks Jonathan