What is the largest true fraction whose unit of fraction is (1/13).
分数单位是(1/13)的最大真分数是多少．
<gadget id="calculator">13 / 1</gadget>
<output>13</output>

<gadget id="calculator">12 / 13</gadget>
<output>12/13 = around 0.923077</output>

<result>12/13 = around 0.923077</result>