When Xiao Ming did addition calculations, he wrote "3" for an "8" that adds tens of digits, and the result is 259. What is the correct answer?
小明做加法计算时，把一个加数十位上的"8写成"3"，结果是259，正确的答案是多少．
<gadget id="calculator">259 + 50</gadget>
<output>309</output>

<result>309</result>