Two people, A and B, are walking in the same direction from cities A and B, which are 36 kilometers apart. B is in front and A is behind. A travels 15 kilometers per hour, and B travels 6 kilometers per hour. How many hours later can A catch up with B?
甲乙两人同时从相距36千米的A、B两城同向而行，乙在前甲在后，甲每小时行15千米，乙每小时行6千米．几小时后甲可追上乙？
<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<gadget id="calculator">36 / 9</gadget>
<output>4</output>

<result>4</result>