Every millimeter is 1/10 of a centimeter, so 1 mm = 0.1 cm and 50 mm = 5 cm. This means that 25 mm can be re-written as 2.5 cm, and we can then find the difference by doing 25 - 2.5 = 22.5 cm. If you were looking for a different type of answer, you can also say that 25 mm is 10 times less than 25 cm.
I hope this helps!