20% of a number is less (1/7) than its (1/2), what is this number?
一个数的20%比它的(1/2)少(1/7)，这个数是多少？
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/2) - (1/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(1/7) / (3/10)</gadget>
<output>10/21 = around 0.47619</output>

<result>10/21 = around 0.47619</result>