From the five natural numbers 3, 13, 17, 29, and 31, take two numbers each time as the numerator and denominator of a fraction, how many simple fractions can be formed in total.
从3、13、17、29、31这五个自然数中，每次取两个数分别作一个分数的分子和分母，一共可组成多少个最简分数．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<result>20</result>