A certain unit of the People’s Liberation Army is marching; the team advances at a speed of 6 kilometers per hour. After 3 hours, the instructor sends a correspondent to catch up with the team at a speed of 15 kilometers per hour on a bicycle to convey the instructions of the superior. How many hours can the correspondent catch up with the team after departure?
解放军某部行军；队伍以每小时6千米的速度前进，3小时后，指导员派通讯员骑自行车以每小时15千米的速度追赶队伍，传达上级指示，通讯员出发后几小时可追上队伍？
<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>