**Solution:**

Since, 1m = 100 cm

Therefore, 8 m 25 cm = 825 cm

6 m 75 cm = 675 cm

4 m 50 cm = 450 cm

HCF(825, 675, 450) will give the length of the longest rod.

825 = 675 x 1 + 150

675 = 150 x 4 + 75

150 = 75 x 2 + 0

Therefore, HCF(825, 675) = 75

450 = 75 x 6 + 0

=> HCF(450, 75) = 75

=> HCF (825, 675, 450) = 75

**Hence, the length of the longest rod is 75 cm.**