Mr. Zhao rode a bicycle for a section of 1,800 meters. One section of the flat road was 200 meters per minute, and the other section was uphill at 80 meters per minute. It took 15 minutes to complete the journey. How many minutes did it take on the flat road?
赵老师骑车行一段1800米的路，其中一段平路每分钟行200米，一段上坡路每分钟行80米，行完全程用了15分钟，平路用了多少分钟？
<gadget id="calculator">1_800 - 1_200</gadget>
<output>600</output>

<gadget id="calculator">600 / 120</gadget>
<output>5</output>

<result>5</result>