As part of the math week celebration, fifth- and sixth-graders decided to build a tower by stacking their math textbooks. For the tower height prediction competition, the students exchanged the measurements of the textbooks. Fifth grade students found that their textbook was 13 mm thick. Sixth grade students found that their textbook was 1.6 cm thick. If 23 fifth grade textbooks were used for the tower and the tower was 57.1 cm tall, how many sixth grade textbooks were used? (Use only the digits 0 - 9 to write the number.)