From the six numbers of 2, 3, 5, 7, 11, and 13, two numbers are randomly selected each time, and they are respectively used as the numerator and denominator of a fraction. How many true fractions are there in total?
从2，3，5，7，11，13六个数中，每次任取2个数，分别作为一个分数的分子和分母，其中真分数共有多少个．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 * (5/2)</gadget>
<output>15</output>

<result>15</result>