`root(4)(64r^3s^4t^5)`

We factor out all the terms that are 4th power. The number `16` is a 4th power, since `2^4= 16`.

`=root(4)((16xx4)r^3s^4(t^4xxt))`

`=root(4)(16s^4t^4)xx(root(4)(4r^3t))`

`=root(4)(2^4s^4t^4)xx(root(4)(4r^3t))`

`=root(4)(2^4)xxroot(4)(s^4)xxroot(4)(t^4)xx(root(4)(4r^3t))`

Then we find the 4th root of each of those terms.

`=2stroot(4)(4r^3t)`

There are no 4th powers left in the expression `4r^3t`, so we leave it under the 4th root sign.