The ratio of number A to number B is 3:5, number B is 20 more than number A, what is number A?
甲数与乙数的比是3：5，乙数比甲数多20，甲数是多少．
<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 * 3</gadget>
<output>30</output>

<result>30</result>