How to filter this signal $x= \sin(2\pi 10 t)+\sin(2 \pi 40 t)+\sin(2 \pi 50 t)+\sin(2 \pi 60 t)$ using comb filters in time domain?
I guess you are trying to design a nortch filter at 50Hz. You can try it in this way.


Generate a signal in frequency domain filter_f=ones(4200,1). 
Then you set the points corresponding to 50Hz and -50Hz to 0.
Transform the signal to time domain.filter=ifftshift(ifft(filter_f)).
window the signal. filter_w=filter.*hamming(4200).
Pickthe center part points of filter_w. And that is the final nortch filter.