At three different locations, the thickness of a glass plate is...

The average thickness of the glass plate is given by:

Average $= \dfrac{2.17 + 2.17 + 2.18}{3}$

Average $= \dfrac{6.52}{3} = 2.1733... \text{ mm}$

Since the original measurements are given up to two decimal places (three significant figures), the final result must also be rounded off to two decimal places (three significant figures) according to the rules of significant figures.

Rounding off $2.1733... \text{ mm}$ to three significant figures, we get $2.17 \text{ mm}$.