Xiao Ming went to climb the mountain on the Double Ninth Festival, and started to climb the mountain at nine o'clock in the morning. He went up the mountain and walked 6 miles per hour, rested at the top for 1.5 hours and began to descend, and walked 7.5 miles per hour. It was already 1:30 in the afternoon when he reached the bottom of the mountain. How many miles did Xiao Ming walk up and down the mountain?
小明在重阳节这天去爬山，上午九点开始爬山．上山每小时走6里，在山顶休息1.5小时开始下山，每小时走7.5里，到山下已经是下午一点半了，小明上下山一共走了多少里．
<gadget id="calculator">4.5 - 1.5</gadget>
<output>3</output>

<gadget id="calculator">5 + 4</gadget>
<output>9</output>

<gadget id="calculator">5 / 9</gadget>
<output>5/9 = around 0.555556</output>

<gadget id="calculator">6 * 3 * (5/9) * 2</gadget>
<output>20</output>

<result>20</result>