A's number is (7/10), subtract B's number to get (7/40), find B's number.
甲数是(7/10)，减去乙数得(7/40)，求乙数．
<gadget id="calculator">7 / 10</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">7 / 40</gadget>
<output>7/40 = around 0.175</output>

<gadget id="calculator">(7/10) - (7/40)</gadget>
<output>21/40 = around 0.525</output>

<result>21/40 = around 0.525</result>