In an exam, the scores of the first group of students happened to constitute an arithmetic sequence with a tolerance of 3, and the total score was 609. Dongdong found that his score was too low. After asking the teacher to correct it, he added 21 points. At this time, they The result is still an arithmetic sequence. I would like to ask: What is the correct score for Dongdong?
在一次考试中，第一组同学的分数恰好构成了公差为3的等差数列，总分为609，冬冬发现自己的分数算少了，找老师更正后，加了21分，这时他们的成绩还是一个等差数列．请问：冬冬正确的分数是多少？
<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">609 / 7</gadget>
<output>87</output>

<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">87 + 12</gadget>
<output>99</output>

<result>99</result>