Ever heard the story of cat and mouse? One day, the cat found a mouse 20 meters ahead, and immediately chased after it. At the same time, the mouse also found the cat, and ran away immediately. The cat runs 7 meters per second and takes 10 seconds to catch up with the mouse. How many meters does the mouse run per second?
听过猫和老鼠的故事吗？一天，猫发现前面20米的地方有只老鼠，立即去追，同时，老鼠也发现了猫，马上就跑．猫每秒跑7米，用了10秒追上老鼠．老鼠每秒跑多少米？
<gadget id="calculator">7 * 10</gadget>
<output>70</output>

<gadget id="calculator">70 - 20</gadget>
<output>50</output>

<gadget id="calculator">50 / 10</gadget>
<output>5</output>

<result>5</result>