A person walks a total of 28.5 kilometers from A to B over the top of the mountain and takes 7 hours and 30 minutes. It is known that the uphill walk is 3 kilometers per hour, and the downhill walk is 5 kilometers per hour. How long will it take him to cross the top of the mountain from place B to return to place A?
某人从甲地上山越过山顶下山到乙地共走了28.5千米，用7小时30分，已知上山每小时走3千米，下山每小时走5千米．他从乙地经原路上山越过山顶返回甲地要用多少时间？
<gadget id="calculator">28.5 / 5</gadget>
<output>5.7</output>

<gadget id="calculator">7.5 - 5.7</gadget>
<output>1.8</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

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

<gadget id="calculator">(1/3) - (1/5)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">1.8 / (2/15)</gadget>
<output>13.5</output>

<gadget id="calculator">28.5 - 13.5</gadget>
<output>15</output>

<gadget id="calculator">15 / 3</gadget>
<output>5</output>

<gadget id="calculator">1.8 / (2/15) / 5</gadget>
<output>2.7</output>

<gadget id="calculator">5 + 2.7</gadget>
<output>7.7</output>

<result>7.7</result>