public void TestGetStatistics()
{
// arrange
var book = new InMemoryBook("", new LetterGrade());
book.AddGrade(88.1);
book.AddGrade(90.6);
book.AddGrade(85.6);
// act
var low = book.GetLow();
var high = book.GetHigh();
var average = book.GetAverage();
// asert
Assert.Equal(85.6, low, 1);
Assert.Equal(90.6, high, 1);
Assert.Equal(88.1, average, 1);
}