Ten firms compete in a market to sell product X. The total sales of all firms selling the product are $2 million.

Ten firms compete in a market to sell product X. The total sales of all firms selling the product are $2 million. Ranking the firms’ sales from highest to lowest, we find the top four firms’ sales to be $260,000, $220,000, $150,000, and $130,000, respectively. Calculate the four-firm concentration ratio in the market for product X?