Xiao Ming goes to climb a mountain on weekends. He drives 3 kilometers per hour when he goes up the mountain, and 5 kilometers per hour when he goes down the mountain. It takes 4 hours to go back and forth. How many kilometers does Xiao Ming need to travel for a round trip?
小明周末去爬山，上山时每小时行驶3千米，下山时每小时行驶5千米，往返一次一共需要4小时．小明往返一次要行多少千米？
<gadget id="calculator">5 + 3</gadget>
<output>8</output>

<gadget id="calculator">5 / 8</gadget>
<output>5/8 = around 0.625</output>

<gadget id="calculator">4 * (5/8)</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">3 * (5/2) * 2</gadget>
<output>15</output>

<result>15</result>