What letter grade is a 75 out of 90?

In a straight scale this is a 60% and is a D-. one point lower ... it would be an E. If the instructor uses a curve or sliding scale it would depend on how well the other students did in comparison. I personally detest curves and sliding scales. They are an indication that the instructor has not been able to 1. motivate the students. 2 properly teach the subject matter or 3 doesn't know how to design a well written test.