The ratio of the two numbers A and B is 4:5, the number B is 20, what is the number A?
甲乙两个数的比是4：5，乙数是20，甲数是多少．
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">20 * (4/5)</gadget>
<output>16</output>

<result>16</result>