A certain boiler burns (2/5) tons of coal in the first week, which is (1/7) less than in the second week, and uses (3/5) less coal in the third week than in the previous two weeks ． How many tons of coal was burned in the third week?
某一锅炉，第一周烧煤(2/5)吨，比第二周少烧煤(1/7)吨，第三周用的煤比前两周所用的煤少(3/5)吨．第三周烧煤多少吨？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

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

<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">(2/5) - (3/5)</gadget>
<output>-1/5 = around -0.2</output>

<gadget id="calculator">(2/5) + (1/7) + (-1/5)</gadget>
<output>12/35 = around 0.342857</output>

<result>12/35 = around 0.342857</result>