There are 7 numbers lined up in a row, their average is 20, the average of the first 5 numbers is 15, and the average of the last 3 numbers is 30, then what is the fifth number equal to
有7个数排成一列，它们的平均数是20，前5个数的平均数是15，后3个数的平均数是30，那么第5个数等于多少
<gadget id="calculator">15 * 5</gadget>
<output>75</output>

<gadget id="calculator">30 * 3</gadget>
<output>90</output>

<gadget id="calculator">20 * 7</gadget>
<output>140</output>

<gadget id="calculator">90 - 140</gadget>
<output>-50</output>

<gadget id="calculator">75 + (-50)</gadget>
<output>25</output>

<result>25</result>