A small brick of sculptor's clay has a mass of 2.2 kilograms. A large brick of sculptor's clay has a mass of 6.4 kilograms. Dante ordered a total of 18 bricks of sculptor's clay. The mass of all the clay was 85.8 kilograms. How many large bricks of clay did Dante order?
Let x be the number of large bricks that Dante ordered.
He ordered 18 -x small bricks.
The mass of the small bricks is 2.2 * (18-x) = 39.6 - 2.2x kg.
The mass of the large bricks is 6.4x kg.
The total mass of the clay is 6.4x + 39.6 - 2.2x = 85.8 kg.
4.2x = 46.2.
x = <<11=11>>11 large bricks. Answer: \boxed{11}.