Sincerely, It is not my homework. It was the question asked in one German University's Online Exams.
I went through Google as well but couldn't find a single way out to figure the solution of this Problem.
A Received Signal is filtered in the low pass filter. At the Input of the Low Pass filter the Bandwidth is B=80khz and the SNR is 30 dB. What is the SNR at output of Filter if equivalent noise bandwidth of the filter (low pass filter) is 20kHz.?