A and B travel from A to B. A starts 30 minutes earlier than B and arrives 30 minutes later. A travels 3.5 kilometers per hour and B travels 4 kilometers per hour. What is the distance between A and B? km?
甲、乙两人由A地到B地，甲比乙早出发30分钟，晚到30分钟，甲每小时走3.5千米，乙每小时走4千米，求A、B两地距离是多少千米？
<gadget id="calculator">4 - 3.5</gadget>
<output>0.5</output>

<gadget id="calculator">4 / 0.5</gadget>
<output>8</output>

<gadget id="calculator">8 * 3.5</gadget>
<output>28</output>

<result>28</result>