Thick candles are the same length as thin candles. A thick candle can be lit for 5 hours, and a thin candle can be lit for 4 hours. If these two candles are lit at the same time, after a period of time, the length of the thick candle is twice that of the thin candle. How long have the two candles been burning?
粗蜡烛和细蜡烛长短一样．粗蜡烛可以点5小时，细蜡烛可以点4小时．同时点燃这两支蜡烛，点了一段时间后，粗蜡烛长是细蜡烛长的2倍．问这两支蜡烛点了多少时间？
<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">2 * (1/4)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/2) - (1/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 / (3/10)</gadget>
<output>10/3 = around 3.333333</output>

<result>10/3 = around 3.333333</result>