30% of a number plus 21 equals itself. What is this number?
一个数的30%加上21等于它本身，这个数是多少？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">21 / (7/10)</gadget>
<output>30</output>

<result>30</result>