A and B start from the same place at the same time to climb a mountain. A goes up the mountain and walks 3 kilometers per hour, descends 4 kilometers per hour, and returns to the original place in 7 hours. It is known that A and B reach the top of the mountain at the same time. When returning, when A has just completed (2/3) of the whole journey up and down the mountain, B has completed (3/4) of the whole journey up and down the mountain. May I ask: How many hours does it take B from departure to return to the original place?
甲、乙两人从同一地点同时出发爬一座山，甲上山每小时走3千米，下山每小时走4千米，7小时返回原地，已知甲、乙两人同时到达山顶，但在返回时，当甲刚走完上山与下山全程的(2/3)，乙已走完上山下山全程的(3/4)．请问：乙从出发到返回原地共用多少小时？
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">7 * (4/7)</gadget>
<output>4</output>

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

<gadget id="calculator">3 * 4 * 2 * (3/4)</gadget>
<output>18</output>

<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">3 * 4 * 2 * (2/3)</gadget>
<output>16</output>

<gadget id="calculator">16 - 12</gadget>
<output>4</output>

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

<gadget id="calculator">4 / 6 / 1</gadget>
<output>2/3 = around 0.666667</output>

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

<gadget id="calculator">2 + 4</gadget>
<output>6</output>

<result>6</result>