A project can be completed 4 days ahead of the specified time by Team A alone, but it can be completed by 5 days beyond the specified time if Team B alone does it. Now that Team A and Team B are working together 4 days later, Team B will do it alone. Completed within the specified time. So how many days is the specified time.
一项工程由甲队单独做可以比规定时间提前4天完成，由乙队单独做则要超过规定时间5天才能完成，现甲、乙两队合做4天后再由乙队独做，正好在规定时间完成．那么规定时间是多少天．
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

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

<gadget id="calculator">20 + 20</gadget>
<output>40</output>

<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">40 / 1</gadget>
<output>40</output>

<result>40</result>