cars arrive according to a poisson distribution with a mean of 80 cars per hr. how long can the car attendant make a phone call if the probability is at least 0.4 that no cars arrive during the call?

my working:

let x be the time that the car attendant can make the call.

then X ~ P(80/60 x)

then P(X = 0) > 0.4 indicates from the statistic table that the mean has to be less than 0.90 and hence

80/60 x = 0.90 means that x = 40 sec

but the answer in my book states that the time is 23 sec.

is there something wrong with my reasoning?