Solve the equation: 80%x-15%x=3.9.
解方程：80%x-15%x=3.9．
<gadget id="calculator">65 / 100</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">3.9 / (13/20)</gadget>
<output>6</output>

<result>6</result>