On Sunday, Xiao Lin and Xiao Gang went to climb the mountain. They walked up the mountain at 2.5 kilometers per hour for 1.5 hours. I went down the mountain without returning the same way, and walked 2.8 kilometers per hour for 1.8 hours. How many kilometers did they walk up and down the mountain?
星期天，小林和小刚去爬山，上山每小时走2.5千米，走了1.5小时．下山没按原路返回，每小时走了2.8千米，走了1.8小时．他们上、下山一共走了多少千米？
<gadget id="calculator">2.5 * 1.5</gadget>
<output>3.75</output>

<gadget id="calculator">2.8 * 1.8</gadget>
<output>5.04</output>

<gadget id="calculator">3.75 + 5.04</gadget>
<output>8.79</output>

<result>8.79</result>