How do you simplify #4 sqrt (81/16)#?
If you recognise 81 and 16 as being 4th powers, the answer is easy to write down as
Otherwise write each number as the product of its prime factors:
The 4th root is easy to find if you remember it is the square root of a square root.
81 and 16 are both perfect squares, but their square roots are also squares.