Solve the equation: (2/7)x+(1/3)x=13.
解方程：(2/7)x+(1/3)x=13．
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(2/7) + (1/3)</gadget>
<output>13/21 = around 0.619048</output>

<gadget id="calculator">13 / (13/21)</gadget>
<output>21</output>

<result>21</result>