There are 3 mountain roads from the foot of the mountain to the top of the mountain. A and B go up the mountain and then return to the foot of the mountain. If two people can go up the mountain in the same way, but they must go down the mountain in different ways, how many combinations are there?
由山脚至山顶有3条山路，甲、乙二人上山后再返回山脚．若两人上山可同路，但下山却必定要不同路，共有多少种组合﹖
<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">9 * 6</gadget>
<output>54</output>

<result>54</result>