Two candles are the same length, the first one can be lit for 4 hours, the second one can be lit for 3 hours, and the two candles are lit at the same time, how many hours later the first one is twice as long as the second one.
两支蜡烛一样长，第一支能点4小时，第二支能点3小时，同时点燃这两支蜡烛，多少小时后第一支是第二支的两倍．
<result>2.4</result>