The ratio of A and B is 3:5, and the difference between the two numbers is 12. What is A's number?
甲、乙两数的比是3：5，两数的差是12，甲数是多少．
<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<result>18</result>