A drives a car, B drives a motorcycle, and A is behind B. If A travels 30 kilometers per hour, he can catch up with B in one hour; if A travels 40 kilometers per hour, he can catch up with B in 30 minutes, then how many kilometers per hour does B (with the same speed) drive?
甲驾驶小汽车，乙驾驶摩托车，甲在乙后面．如果甲每小时行驶30公里，1小时可以追上乙；如果甲每小时行驶40公里，30分钟可以追上乙，那么乙(速度始终不变)每小时行驶多少公里．
<gadget id="calculator">0.5 * 40</gadget>
<output>20</output>

<gadget id="calculator">30 - 20</gadget>
<output>10</output>

<gadget id="calculator">1 - 0.5</gadget>
<output>0.5</output>

<gadget id="calculator">10 / 0.5</gadget>
<output>20</output>

<result>20</result>