20 unequal natural numbers, their average is 15.5, what is the largest number among these natural numbers?
20个互不相等的自然数，它们的平均数是15.5，这些自然数中最大的数是多少．
<gadget id="calculator">15.5 * 20</gadget>
<output>310</output>

<gadget id="calculator">0 + 18</gadget>
<output>18</output>

<gadget id="calculator">19 / 2</gadget>
<output>19/2 = around 9.5</output>

<gadget id="calculator">18 * (19/2)</gadget>
<output>171</output>

<gadget id="calculator">310 - 171</gadget>
<output>139</output>

<result>139</result>