A's number is 60, which is 20 more than B's (4/5), find B's number.
甲数是60，比乙数的(4/5)多20，求乙数．
<gadget id="calculator">60 - 20</gadget>
<output>40</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">40 / (4/5)</gadget>
<output>50</output>

<result>50</result>