The distance between places A and B is 20 kilometers. There are 45 students in a class, and they go from place A to place B. There are a batch of horse-drawn carriages, the speed of which is three times the speed of a person walking. The carriage can take 9 people at a time. The first batch of 9 students will be sent to B at A, and the rest of the students will walk to B at the same time; Immediately return after reaching point B. After meeting walking students on the way, pick up 9 students and send them to point B, and the rest of the students continue to move forward to point B; ...the same multiple round trips. When all the students arrive at point B, how many kilometers has the carriage traveled in total?
A、B两地相距20千米，一个班有学生45人，由A地去B地．现有一批马车，车速是人步行速度的3倍，马车每次可乘坐9人，在A地先将第一批9名学生送往B地，其余学生同时步行向B地前进；车到B地后立即返回，在途中与步行学生相遇后，再接9名学生送往B地，余下学生继续向B地前进；…同样多次往返．当全体学生都到B地时，马车共行了多少千米．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">20 * (2/3) * (3/4) * 2</gadget>
<output>20</output>

<gadget id="calculator">20 * (2/3) * (3/4) * (2/3) * (3/4) * 2</gadget>
<output>10</output>

<gadget id="calculator">2.5 * 2</gadget>
<output>5</output>

<gadget id="calculator">1.25 * 2</gadget>
<output>2.5</output>

<gadget id="calculator">20 + 20 + 10 + 5 + 2.5</gadget>
<output>57.5</output>

<result>57.5</result>