Subtract a number from the difference between (9/10) and (2/5) to get (1/4). What is this number? (solution by equation)
从(9/10)与(2/5)的差里减去一个数得(1/4)，这个数是多少？(用方程解)
<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">(9/10) - (1/4) - (2/5)</gadget>
<output>1/4 = around 0.25</output>

<result>1/4 = around 0.25</result>