4. It is believed that the number of sets of traffic lights that fail per hour in a particular large city follows a Poisson distribution with a mean of 3 . Find the probability that

1. there will be no failures in a one-hour period,
2. there will be more than 4 failures in a 30 -minute period. Using a suitable approximation, find the probability that in a 24-hour period there will be
3. less than 60 failures,
4. exactly 72 failures.