Solution ratio: (7/(4.2))=((x+7)/9).
解比例：(7/(4.2))=((x+7)/9)．
<gadget id="calculator">7 / 9</gadget>
<output>7/9 = around 0.777778</output>

<gadget id="calculator">(7/9) * 4.2</gadget>
<output>3.266667</output>

<gadget id="calculator">7 - 3.266667</gadget>
<output>3.733333</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/9) * 4.2</gadget>
<output>0.466667</output>

<gadget id="calculator">3.733333 / 0.466667</gadget>
<output>7.999994</output>

<result>7.999994</result>