Given a wooden log of diameter 2 *ch'ih* 5 *ts'un* from which a 7 *ts'un* thick board is to be cut, what is the maximum possible width of the board? [Note: 1 *ch'ih* = 10 *ts'un*]

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

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The answer is: 2 *ch'ih* 4 *ts'un*