In a proportional formula, the two internal terms are reciprocals of each other, one external term is (4/15), and the other external term is how much.
在一个比例式中，两内项互为倒数，一个外项是(4/15)，另一个外项是多少．
<gadget id="calculator">4 / 15</gadget>
<output>4/15 = around 0.266667</output>

<gadget id="calculator">1 / (4/15)</gadget>
<output>15/4 = around 3.75</output>

<result>15/4 = around 3.75</result>