Final answer:
To find the diameter of the coaster, subtract twice the extension from the diameter of the glass.
Step-by-step explanation:
To find the diameter of the coaster, we need to subtract twice the extension from the diameter of the glass. Let's assume the diameter of the glass is D cm. Since the coaster extends beyond the edge of the glass by 1 cm on each side, the diameter of the coaster will be D - 2 cm.
Therefore, the diameter of the coaster is D - 2 cm.