The lifetimes of bulbs of a particular type have an exponential distribution with parameter . Five bulbs of this type are switched on at time Their life- λ [h− 1 ] t = 0. times can be assumed...

The lifetimes of bulbs of a particular type have an exponential distribution with parameter . Five bulbs of this type are switched on at time Their life- λ [h−¹] t = 0. times can be assumed independent.

(1) What is the probability that at time a) all 5, b) at least 3 bulbs are failed? t = 1/λ

(2) What is the probability that at least one bulb survives hours?