The teachers and students of a certain school carry out marching activities and advance at a speed of 6 kilometers per hour. After 3 hours, the school sends a messenger to ride the same road on a bicycle to deliver orders. If the messenger chases the team at a speed of 15 kilometers per hour, How many hours will it take to catch up?
某校师生开展行军活动，以每小时6千米的速度前进，3小时后学校派通迅员骑自行车走同一条路去传达命令，如果通讯员以每小时15千米的速度去追赶队伍，需要多少小时才能赶上？
<gadget id="calculator">3 * 6</gadget>
<output>18</output>

<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<gadget id="calculator">18 / 9</gadget>
<output>2</output>

<result>2</result>