A fraction, after adding 1 to the numerator, its value is 1, and after subtracting 1 from the numerator, its value is (4/5), what is this fraction?
一个分数，分子加上1后，其值为1，分子减去1后，其值为(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>9/5 = around 1.8</output>

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

<gadget id="calculator">(9/5) / (1/5)</gadget>
<output>9</output>

<gadget id="calculator">9 + 1</gadget>
<output>10</output>

<gadget id="calculator">(9/5) / (1/5) / 10</gadget>
<output>9/10 = around 0.9</output>

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