A and B start from the same place in the same direction, A travels 7 kilometers per hour, and B travels 5 kilometers per hour. B walks for 2 hours before A starts walking. How long does it take for A to catch up with B?
甲、乙二人同地同方向出发，甲每小时走7千米，乙每小时走5千米．乙先走2小时后，甲才开始走，甲追上乙需要几小时？
<gadget id="calculator">7 - 5</gadget>
<output>2</output>

<gadget id="calculator">2 / 2</gadget>
<output>1</output>

<gadget id="calculator">5 * 1</gadget>
<output>5</output>

<result>5</result>