A car goes back and forth between A and B. It takes 4 hours to go, and the speed increases (1/7) when it comes back. How many hours does it take to come back?
一辆车往返甲乙，去时用4小时，回来时，速度提高了(1/7)，回来时用多少小时．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">1 + (1/7)</gadget>
<output>8/7 = around 1.142857</output>

<gadget id="calculator">(1/4) * (8/7)</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">1 / (2/7)</gadget>
<output>7/2 = around 3.5</output>

<result>7/2 = around 3.5</result>