The ratio of two numbers A and B is 4:5, B is 15, what is A.
A、B两个数的比是4：5，B是15，A是多少．
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">15 * (4/5)</gadget>
<output>12</output>

<result>12</result>