fraction, if you add a fraction unit to it, you get 1, and if you subtract a fraction unit, you get (4/5). What is the fraction?
一个分数，如果加上它的一个分数单位就得1，如果减去一个分数单位就得(4/5)，这个分数是多少．
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

<result>9/10 = around 0.9</result>