A communication system always encounters one of the three possible interference waveforms: F1, F2 or F3. The probability of each interference is 0.8, 0.16 and 0.04, respectively. The communication system fails with probabilities 0.01, 0.1, 0.4 when it encounters F1, F2 and F3, respectively. Given the system has failed, find the probability that the failure is a result of F1, F2 or F3, respectively.