There is a light pole 50 meters apart on one side of the road. Xiaojun took the trolley bus for 2 minutes and saw 21 light poles on one side of the road. He asked how many kilometers the tram traveled per hour.
公路的一边相隔50米有一根路灯杆，小军乘无轨电车2分钟看到马路的一边有路灯杆21根，问电车每小时行多少千米．
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">20 / 1_000 / 30</gadget>
<output>1/1_500 = around 0.000667</output>

<gadget id="calculator">50 * (1/1_500)</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">20 / 1_000 / (1/30)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">50 * (3/5)</gadget>
<output>30</output>

<result>30</result>