The dog days of summer took a toll on luxury real estate last week, with just 21 contracts signed on properties $4 million and up, according to Olshan Realty’s latest luxury report. Of homes that went into contract, the average asking price was $6.8 million; properties spent an average of 350 days on the market. The top sale of the week was an 11-room penthouse at 240 Riverside Boulevard, asking $15.75 million, down from the asking price of $18.5 million when the apartment first hit the market in March 2012. Measuring 5,168 square feet, the condo has five bedrooms, 4.5 bathrooms, 11-foot ceilings and views of the Hudson River and Central Park.

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