It is known that the sum of A and B is 36, and the ratio of A and B is 4:5. What is the number of B?
已知甲、乙两数的和是36，甲与乙的比是4：5，乙数是多少．
<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">36 / 9</gadget>
<output>4</output>

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

<result>20</result>