If the product of 4 and 5 is subtracted from half of a number, the difference is 3.5. Find this number.
一个数的一半减去4与5的积，差是3.5，求这个数．
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">20 + 3.5</gadget>
<output>23.5</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">23.5 / (1/2)</gadget>
<output>47</output>

<result>47</result>