Thirty-ninth

He knew that Mr. Welland, who was behaving "very handsomely," already had his eye on a newly built house in East Thirty-ninth Street. The neighbourhood was thought remote, and the house was built in a ghastly greenish-yellow stone that the younger architects were beginning to employ as a protest against the brownstone of which the uniform hue coated New York like a cold chocolate sauce; but the plumbing was perfect.

The answer appears on the thirty-ninth page of the book.

She finished thirty-ninth in the race.

A thirty-ninth of 780 is 20.