manuscript needs to be typed alone, A needs 12 hours, B needs 20 hours, now A will type for 3 hours first, B will type for 4 hours, how much is left to be typed.
一份稿件，单独打完，甲要12小时，乙要20小时，现在甲先打3小时，乙接着打4小时，还剩下多少没有打完．
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 3</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 4</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/4) - (1/5)</gadget>
<output>11/20 = around 0.55</output>

<result>11/20 = around 0.55</result>