Add (2/5) to (5/6) of a number, and the sum is (9/10). What is this number?
一个数的(5/6)加上(2/5)，和是(9/10)．这个数是多少？
<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 / 6</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">(1/2) / (5/6)</gadget>
<output>3/5 = around 0.6</output>

<result>3/5 = around 0.6</result>