What is the ending inventory in the following case?

What is the ending inventory in the following case? A company that has operated with a 30% average gross profit ratio for a number of years had $100,000 in sales during the first quarter of this year. If it began the quarter with $18,000 of inventory at cost and purchased $72,000 of inventory during the quarter, its estimated ending inventory using the gross profit method is: a. $30,000 b. $21,000 c. $20,000 d. $18,000 e. $27,000