public void BookReturnsAverageGrade()
{
// arrange
var book = new InMemoryBook("Test Book");
book.AddGrade(99.9);
book.AddGrade(49.3);
book.AddGrade(39.3);
book.AddGrade(29.3);
book.AddGrade(19.3);
var expected = 47.42;
// act
var actual = book.GetGradeAverage();
// assert
Assert.Equal(actual, expected);
}