The test scores for Mrs. Barrett's class are shown below
12, 23, 36, 21, 42, 55, 19, 35, 36, 29, 11, 19
To find the average (mean) test score for Mrs. Barrett's class, we add up all the scores and divide by the number of scores:
12 + 23 + 36 + 21 + 42 + 55 + 19 + 35 + 36 + 29 + 11 + 19 = 318
There are 12 scores in total, so we divide by 12:
318 รท 12 = 26.5
Therefore, the average test score for Mrs. Barrett's class is 26.5.