A simplest false fraction, the product of numerator and denominator is 21, and both are prime numbers, what is the fraction.
一个最简假分数，分子、分母的积是21，且都是质数，这个分数是多少．
<gadget id="calculator">7 / 3</gadget>
<output>7/3 = around 2.333333</output>

<result>7/3 = around 2.333333</result>