If Xiao Ming yelled at the opposite mountain on the top of the mountain, he heard the echo from the opposite side about 4 seconds later. It is known that the speed of sound in the air is about 340 meters per second. How many kilometers is the distance between the tops of these two mountains?
如果小明在山顶对着对面的大山大吼一声，大约4秒钟后听到了对面的回声．已知声音在空气中的传播速度大约是每秒340米，这两座山的山顶之间大约相距多少千米？
<gadget id="calculator">4 / 2 / 1_000</gadget>
<output>1/500 = around 0.002</output>

<gadget id="calculator">340 * (1/500)</gadget>
<output>17/25 = around 0.68</output>

<result>17/25 = around 0.68</result>