How do you simplify #sqrt125 + sqrt80#?
The result is
When you want to simplify the roots, the best thing you can do is to write the factors of your numbers. In your case
The square root is an operation that if you have the same numbers, gives you back one of the two numbers.
As an example
So we apply this property of the square root grouping the numbers in pairs (when possible) and removing from the square root:
Here I grouped a pair of
Again I extracted the pairs from the root.
Finally I have: