If you mean 125^(2/3) it's 25.
It's the cube root of 125 (which is 5 ) which you then square to get 25.
Another example to help you understand - 16^(3/4) means 'take the fourth root of 16 (which is 2) and then cube it ' (to get 8)
One more. 27^(2/3) means 'take the cube root of 27 (which is 3) and square it' (to get 9)
