There are 50 students in Class 5 of the fourth grade. After a math test, Mr. Wang ranked these students according to their grades, and found that the average score of the top 30 students was 12 points higher than the average score of the bottom 20 students. A classmate was not clear about the concept of "average". He added the average score of the top 30 to the average score of the bottom 20, and then divided it by 2. He mistakenly thought that this was the average score of the whole class. Has your average grade increased or decreased? Please calculate how many points have been improved or decreased?
四年级五班有50名同学，在一次数学考试后，王老师把这些学生按成绩排了名次，发现前30名的平均分比后20名的平均分多12分．一位同学对"平均"的概念不清楚，他把前30名的平均分加上后20名的平均分，再除以2，错误地认为这就是全班同学的平均分，这样做全班的平均成绩是提高了，还是降低了？请算出提高或降低了多少分？
<gadget id="calculator">60 * 20</gadget>
<output>1_200</output>

<gadget id="calculator">60 + 12</gadget>
<output>72</output>

<gadget id="calculator">72 * 30</gadget>
<output>2_160</output>

<gadget id="calculator">1_200 + 2_160</gadget>
<output>3_360</output>

<gadget id="calculator">3_360 / 50</gadget>
<output>336/5 = around 67.2</output>

<gadget id="calculator">60 + 72</gadget>
<output>132</output>

<gadget id="calculator">132 / 2</gadget>
<output>66</output>

<gadget id="calculator">(336/5) - 66</gadget>
<output>6/5 = around 1.2</output>

<result>6/5 = around 1.2</result>