The dimensions of room are
Length = 8 m 25 cm = 825 cm
Breadth = 6 m 75 cm = 675 cm
Height = 4 m 50 cm = 450 cm
So the longest rod = HCF of 825, 675 and 450
We know that the prime factorization of 825=3×5×5×11
The same way prime factorization of 675=3×3×3×5×5
Prime factorization of 450=2×3×3×5×5
So the HCF of 825, 675 and 450=3×5×5=75
Therefore, the longest rod which can measure the dimensions of the room exactly is 75 cm.