The horse ran 3 steps in the time that the dog ran 5 steps, and the horse ran 4 steps away from the dog's 7 steps. Now the dog has run 30 meters, and the horse starts to chase it. As long as the dog runs, the horse can catch it.
狗跑5步的时间马跑3步，马跑4步的距离狗跑7步，现在狗已跑出30米，马开始追它．狗再跑多少，马可以追到它．
<gadget id="calculator">21 - 20</gadget>
<output>1</output>

<gadget id="calculator">30 / 1</gadget>
<output>30</output>

<gadget id="calculator">30 * 21</gadget>
<output>630</output>

<gadget id="calculator">630 - 30</gadget>
<output>600</output>

<result>600</result>