In a math class that contains both 11th and 12th graders, each student must do a class presentation on a famous mathematician. Each student may do the presentation alone or with a class partner. An 11th grader's partner must be a 12th grader and a 12th grader's partner must be an 11th grader. If two-thirds of the 11th graders and three-fifths of the 12th graders work with partners, what percentage of the class works alone? Round your answer to the nearest whole number.