A pile of yellow sand was transported to the construction site, and after (4/7) tons were used, there were still (2/7) tons left. How many tons is this pile of yellow sand?
工地上运来一堆黄沙，用去(4/7)吨后，还剩(2/7)吨．这堆黄沙原来有多少吨？
<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">(4/7) + (2/7)</gadget>
<output>6/7 = around 0.857143</output>

<result>6/7 = around 0.857143</result>