Sampling $x(t)=\cos(4\pi t)+\cos(2\pi t)$ Imagine that we sample the signal $x(t)=\cos(4\pi t)+\cos(2\pi t)$ with a certain sample frequency $f_s$ and we obtain $x[n]$. Now, by ideal interpolation, we get $y(t)$ from $x[n]$.

How can we know the used sample frequency $f_s$ by looking at $y(t)?$

As a particular case, which $f_s$ verifies that $y(t)=2\cos(2\pi t)?$
How can we know the used sample frequency fs by looking at y(t)?


You can't. 

The sampling theorem states that any sampling higher than twice the highest signal frequency allows for perfect reconstruction. In you case any sampling frequency higher than 4 would result in the same $y(t)$.