GraderService.cs

namespace Grader.Service
{
    public class GradeConverter
    {
        /// <summary>
        /// 90 and above = 'A'
        /// 80 - 90 = 'B'
        /// 70 - 80 = 'C'
        /// 60 - 70 = 'D'
        /// 60 and below = 'F'
        /// </summary>
        /// <param name="grade"></param>
        /// <returns>Letter grade</returns>
        public char GetLetterGrade(int grade) 
        {
            if (grade > 90)
            {
                return 'A';
            }
            if (grade < 90 && grade > 80)
            {
                return 'B';
            }
            if (grade < 80 && grade > 70)
            {
                return 'C';
            }
            if (grade < 70 && grade > 60)
            {
                return 'D';
            }
            if (grade < 60)
            {
                return 'F';
            }

            return 'F';
        }
    }
}