A rope is 18m long, if it is used for the first time (1/3), how many meters will be used again is exactly (1/2) of this rope.
一根绳子长18m，第一次用去(1/3)，再用多少米正好是这根绳子的(1/2)．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/2) - (1/3)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">18 * (1/6)</gadget>
<output>3</output>

<result>3</result>