A​ student's course grade is based on one midterm that counts as 5​% of his final​ grade, one class project that counts as 10​% of his final​ grade, a set of homework assignments that counts as 45​% of his final​ grade, and a final exam that counts as 40​% of his final grade. His midterm score is 78​, his project score is 93​, his homework score is 94​, and his final exam score is 64. What is his overall final​ score? What letter grade did he earn​ (A, B,​ C, D, or​ F)? Assume that a mean of 90 or above is an​ A, a mean of at least 80 but less than 90 is a​ B, and so on.