4 The number of emergency telephone calls to the electricity board office in a certain area in \(t\) minutes is known to follow a Poisson distribution with mean \(\frac { 1 } { 80 } t\).
- Find the probability that there will be at least 3 emergency telephone calls to the office in any 20-minute period.
- The probability that no emergency telephone call is made to the office in a period of \(k\) minutes is 0.9 . Find \(k\).