1 - (1/30 + x) = 29/30 - xWe have also been told that the probability of false alarm is 10x. Hence the probability of real alarm is 1/30 = 10x. But this is equal to 3x since 3 times as many sharks are detected as go undetected
1/30 - 10x = 3xHence x = 1/390
Thus the probability of a peaceful day is
29/30 - 1/390 = 0.964