From A to B, Xiao Ming walks 5 kilometers per hour when he goes there, and 7 kilometers per hour when he comes back. It takes 4 hours to go back and forth. How long did it take Xiao Ming to go?
小明从甲地到乙地，去时每小时走5千米，回来时每小时走7千米，来回共用4小时，小明去时用了多长时间？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">(1/5) + (1/7)</gadget>
<output>12/35 = around 0.342857</output>

<gadget id="calculator">4 / (12/35) / 5</gadget>
<output>7/3 = around 2.333333</output>

<result>7/3 = around 2.333333</result>