On the top, the cube root of 1000 is 10.

On the botoom of the fraction, we have a negative index. We need to recall these 2 properties of fractions and negative indices:

If the number on the bottom has a negative index, then this is the result:

So 400^−1/2 on the bottom becomes 400^1/2 on the top.

And 400^1/2 = √400 = 20.

Here's the complete answer: