Xiao Chen climbed over the top of the mountain from point A to point B, and traveled a total of 30.5 kilometers. It took 7 hours. His uphill speed was 4 kilometers per hour, and his downhill speed was 5 kilometers per hour. If the uphill and downhill speeds are not How long does it take to return from point B to point A?
小陈从A地翻过山顶到B地，共行了30.5千米，用了7个小时，他上山速度为每小时4千米，下山速度为每小时5千米，如果上山、下山速度不变，由B地返回A地要用多少时间？
<gadget id="calculator">30.5 / 5</gadget>
<output>6.1</output>

<gadget id="calculator">7 - 6.1</gadget>
<output>0.9</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">0.9 / (1/20) / 5</gadget>
<output>3.6</output>

<gadget id="calculator">0.9 / (1/20)</gadget>
<output>18</output>

<gadget id="calculator">30.5 - 18</gadget>
<output>12.5</output>

<gadget id="calculator">12.5 / 4</gadget>
<output>3.125</output>

<gadget id="calculator">3.6 + 3.125</gadget>
<output>6.725</output>

<result>6.725</result>