For a project, it takes 4 days longer for A to do it alone than A and B’s cooperation, and 9 days longer for B to do it alone than A’s and B’s cooperation. How many days does B need to work alone?
一项工程，甲单独做比甲，乙两人合作多用4天，乙单独做比甲，乙两人合作多用9天，问乙单独需要几天？
<gadget id="calculator">6 + 9</gadget>
<output>15</output>

<result>15</result>