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. Question: How far can the dog run before the horse can overtake it? How many.
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．问：狗再跑多远，马可以追上它？多少．
<gadget id="calculator">21 - 20</gadget>
<output>1</output>

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

<gadget id="calculator">20 * 30</gadget>
<output>600</output>

<result>600</result>