how to compute the signal passing from the low pass filter?
Low pass filters get the name because they pass signals at low frequencies and attenuate signals at higher frequencies. The definition of low and high depend on the the cut-off frequency of the filter. In your case, the cut-off frequency is $\frac{\pi}{5}$. Your filter is the ideal low pass filter where signals above the cut-off frequency are completely zeroed out. The input signal is a perfect cosine signal so all of its energy is concentrated at one specific frequency. It happens to be at a frequency above the cut-off frequency, so what should happen?