def convert_to_letter(grade):
    """Convert a decimal number to letter grade"""
    grade = round(grade, 1)
    if grade >= 82.5:
        return 'A'
    elif grade >= 65:
        return 'B'
    elif grade >= 55:
        return 'C'
    elif grade >= 50:
        return 'D'
    else:
        return 'F'