Okay, so I need to find the class average for the test. Let me think about how to approach this. The average is the mean, right? Which means I need to add up all the students' scores and then divide by the number of students. 

First, let's break down the information given. There are 30 students in total. Out of these, 20 scored 80, 8 scored 90, and 2 scored 100. Hmm, let me check if those numbers add up to 30. 20 plus 8 is 28, plus 2 more makes 30. Okay, that checks out. 

So, to find the total sum of all scores, I need to multiply the number of students who got each score by the score itself and then add those products together. Let me write that down step by step. 

First group: 20 students with 80 each. So 20 times 80. Let me calculate that. 20 times 80 is 1600. 

Second group: 8 students with 90 each. 8 times 90. Hmm, 8 times 90... 8*9 is 72, so 8*90 is 720. 

Third group: 2 students with 100 each. 2 times 100 is 200. 

Now, add all those totals together. 1600 plus 720 is 2320. Then 2320 plus 200 is 2520. So the total sum of all scores is 2520. 

Now, to find the average, I divide this total by the number of students, which is 30. So 2520 divided by 30. Let me do that division. 

2520 divided by 30. Well, 30 times 80 is 2400. Subtract 2400 from 2520, that leaves 120. Then 120 divided by 30 is 4. So total is 80 plus 4, which is 84. 

Wait, let me verify that again. 30 times 84 is 2520. Because 30 times 80 is 2400, and 30 times 4 is 120, so 2400 + 120 = 2520. Yep, that's correct. 

So the class average is 84. 

But just to make sure I didn't make any calculation errors, let me go through the steps again. 

Total score: 20*80 = 1600, 8*90=720, 2*100=200. Total sum is 1600 + 720 + 200. Let's add 1600 and 720 first. 1600 + 700 is 2300, plus 20 is 2320. Then 2320 + 200 = 2520. Correct. 

Divide by 30: 2520 ÷ 30. Let me divide both numerator and denominator by 10 first to simplify: 252 ÷ 3. 3 times 80 is 240, remainder 12. 12 ÷ 3 is 4. So 80 + 4 = 84. Yep, same result. 

Alternatively, 30 into 2520. 30*80=2400, 2520-2400=120. 120/30=4. So total 84. 

Everything checks out. Therefore, the class average is 84.
Therefore, the class average on the test is $\boxed{84}$.
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