Matt measures 8 cells in with micrometers. Furthermore, he measures 12 cell with 2 micrometer.
In order to calculate the average measurements of the cells Matt had, you take into account that the total number of cells is 20. Then, you have:
avg = (8(3) + 12(2))/ (8+12) = (24 + 24)/20 = 48/20
avg = 24/10 = 12/5 = 2.4
Then, the average of the measurements of Matt was 2.4 micrometers