Calls for dial-in connections to a computer centre arrive at an average rate of four per minute. The calls follow a Poisson distribution. If a call arrives at the beginning of a 1-minute interval, what is the probability that a second call will not arrive in the next 20 seconds?

I'm having trouble putting this into the Poisson probability distribution formula. It seems different from some of the other problems I've done. How do you solve this?

Jack