A is 28 kilometers behind B, and the two are walking in the same direction at the same time. A travels 16 kilometers per hour, and B travels 9 kilometers per hour. How long does it take for A to catch up with B?
甲在乙的后面28千米，两人同时同向而行，甲每小时行16千米，乙每小时行9千米，甲几小时追上乙？
<gadget id="calculator">16 - 9</gadget>
<output>7</output>

<gadget id="calculator">28 / 7</gadget>
<output>4</output>

<result>4</result>