### Solution
#### 1.1 Sample Size
The sample size refers to the total number of rats used in this investigation.
We have three groups (A, B, and C), and each group consists of 3 rats. Therefore, the total sample size is calculated as:
[tex]\[ 3 \text{ groups} \times 3 \text{ rats per group} = 9 \text{ rats} \][/tex]
So, the sample size of this investigation is 9.
#### 1.2 Average Mass Calculation
We will calculate the average mass for each group (i.e., (i), (ii), and (iii)) by summing the masses of the three rats in each group and then dividing by the number of rats.
For Group A:
The masses of the three rats in Group A are:
[tex]\[ 320 \, \text{g}, \, 322 \, \text{g}, \, 322 \, \text{g} \][/tex]
To find the average mass of Group A (i):
[tex]\[ \text{Average mass of Group A (i)} = \frac{320 + 322 + 322}{3} = \frac{964}{3} = 321.33 \, \text{g} \][/tex]
For Group B:
The masses of the three rats in Group B are:
[tex]\[ 305 \, \text{g}, \, 315 \, \text{g}, \, 308 \, \text{g} \][/tex]
To find the average mass of Group B (ii):
[tex]\[ \text{Average mass of Group B (ii)} = \frac{305 + 315 + 308}{3} = \frac{928}{3} = 309.33 \, \text{g} \][/tex]
For Group C:
The masses of the three rats in Group C are:
[tex]\[ 345 \, \text{g}, \, 338 \, \text{g}, \, 337 \, \text{g} \][/tex]
To find the average mass of Group C (iii):
[tex]\[ \text{Average mass of Group C (iii)} = \frac{345 + 338 + 337}{3} = \frac{1020}{3} = 340.00 \, \text{g} \][/tex]
### Summary of Results
1. Sample size: 9
2. Average mass of Group A (i): 321.33 g
3. Average mass of Group B (ii): 309.33 g
4. Average mass of Group C (iii): 340.00 g
These are the detailed steps and results for calculating the sample size and the average masses for each group in this investigation.
