For a piece of cloth, if (2/5) of it is used, there will be 15 meters left; if (3/10) of it is used, how many meters will be used.
一段布，用去它的(2/5)，就剩下15米；若用去它的(3/10)，则用去多少米．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">15 / (3/5)</gadget>
<output>25</output>

<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">25 * (3/10)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>