The two internal terms of a ratio are reciprocals of each other, one of the external terms is 15, and the other external term is how much.
一个比例的两个内项互为倒数，其中一个外项是15，另一个外项是多少．
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<result>1/15 = around 0.066667</result>