Add (2/5) to (5/7) of a number, and the sum is (9/10). Find this number.
一个数的(5/7)加上了(2/5)，和是(9/10)，求这个数．
<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(9/10) - (2/5)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">(1/2) / (5/7)</gadget>
<output>7/10 = around 0.7</output>

<result>7/10 = around 0.7</result>