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The Door is Ajar

A two door gate of unknown width is opened so that a 2 ts'un gap exists between the two doors.  It is known that the open door's edge protrudes 1 ch'ih from the door sill.  What is the width of the gate? [Note: 1 ch'ih = 10 ts'un]

Jiuzhang Suanshu (The Nine Chapters on the Mathematical Art), c. 100 BCE

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"The Door is Ajar," Convergence (July 2006)