simple fraction, plus its fractional unit, results in (1/2); reducing its fractional unit equals (2/5). What is the original number.
一个最简分数，加上它的分数单位，结果是(1/2)；减少它的分数单位等于(2/5)．原数是多少．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(1/2) - (2/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) / 2</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/2) - (1/20)</gadget>
<output>9/20 = around 0.45</output>

<result>9/20 = around 0.45</result>