In the mixing of concrete both the size of the gravel and thickness of the mortar can possibly influence the hardness of the resulting concrete 3 levels of gravel size (factor A) and 3 levels of mortar thickness (factor B) were examined in a carefully controlled experiment. For each combination of gravel size and mortar thickness 4 batches of concrete were mixed and tested for hardness with results shown below. Size of gravel (1) Small Medium large Mortar thickness (1) Light 3 4 12 11 5 5 13 9 7 5 3 6 Medium 3 346ssa 4 6 8 11 5 5 9 4 4326 2 6 Heavy 16 14 7 10 16 7 9 19 12 4 11 5 c. Perform Duncan's multiple range test on the average of size of gravel for small, medium and large at the 5% level.