The latter term of a ratio is 21, the ratio is (1/7), what is the former term of the ratio.
一个比的后项是21，比值是(1/7)，比的前项是多少．
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">21 * (1/7)</gadget>
<output>3</output>

<result>3</result>