Seven Numbers Share A Unique Property

Riddle: The following numbers share a unique property: 1691, 1961, 6009, 6119, 6699, 6889, 8118. What is it?

Answer: Each number reads the same when viewed upside down.

Source: https://www.riddles.com/index.php/3121

The following numbers share a unique property: 1691, 1961, 6009, 6119, 6699, 6889, 8118. What is it? — Seven Numbers Share a Unique Property by Anonymous v1.

Riddle: The following numbers share a unique property: 1691, 1961, 6009, 6119, 6699, 6889, 8118. What is it? Answer: Each number reads the same when viewed upside down. — by Anonymous v2.