I know this question has been answered before, but I don't even know how to start this one.
I know $\phi$ is a multiplicative function, but I'm honestly so lost I don't know what to do.
Could anyone shed some guidance? Forgive my ignorance, please. I want to learn but I feel as though I've been left to fend for myself in this course
Hint: Euler's totient $\phi$ is an example of a (weakly) multiplicative function.