Two people, A and B, start from A to B at the same time. If they both travel at the same speed, A takes 4 hours to complete the journey, and B takes 6 hours to complete the journey. Then when B's remaining distance is four times that of A's remaining distance, how many hours have they set off?
甲、乙两人同时从A地出发到B地，若两人都匀速行进，甲用4小时走完全程，乙用6小时走完全程．则当乙所剩路程是甲所剩路程的4倍时，他们已经出发了多少小时．
<gadget id="calculator">4 / 10</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">9 * (2/5)</gadget>
<output>18/5 = around 3.6</output>

<result>18/5 = around 3.6</result>