public void bookCalculatesAnAverageGrade()
{
//arrange
var book = new InMemoryBook("");
book.AddGrade(89.1);
book.AddGrade(90.5);
book.AddGrade(77.3);
//act
var result = book.Getstat();
//assert
Assert.Equal(85.6, result.average, 1);
Assert.Equal(90.5, result.high, 1);
Assert.Equal(77.3, result.low, 1);
Assert.Equal('B', result.letter);
}