The thicknes of a glass plate is measured to be 2.17 and 2.17 mm and 2.18 mm at three different places. Find the average thickness of the plate from this data.

Explanation

To find the average thickness of the glass plate from the given measurements, we can follow these steps:

### Step 1: List the Measurements
We have three measurements of the thickness of the glass plate:
- \( t_1 = 2.17 \, \text{mm} \)
- \( t_2 = 2.17 \, \text{mm} \)
- \( t_3 = 2.18 \, \text{mm} \)

### Step 2: Sum the Measurements
Next, we will sum these three measurements:
\[ \text{Sum} = t_1 + t_2 + t_3 = 2.17 + 2.17 + 2.18 \]

Calculating the sum:
\[ \text{Sum} = 2.17 + 2.17 + 2.18 = 6.52 \, \text{mm} \]

### Step 3: Calculate the Average
Now, we will calculate the average thickness by dividing the sum by the number of measurements (which is 3):
\[ \text{Average Thickness} = \frac{\text{Sum}}{3} = \frac{6.52}{3} \]

Calculating the average:
\[ \text{Average Thickness} = 2.1733 \, \text{mm} \]

### Step 4: Round to Significant Figures
Since all the measurements have 3 significant figures, we need to round our average to 3 significant figures. The average thickness is:
\[ 2.1733 \, \text{mm} \]
Looking at the third digit (7), the next digit (3) is less than 5, so we do not round up. Thus, we round it to:
\[ \text{Average Thickness} = 2.17 \, \text{mm} \]

### Final Answer
The average thickness of the glass plate is:
\[ \boxed{2.17 \, \text{mm}} \]