can you explain me the part A

Let us assume we have two legitimate users1 A and B1 in a computer systems who are known to type characters on a keyboard according to a. Poisson process with rates A E {300,600} characters per minute. We are interested to see how reliable a spy who is listening to the sound of typing can infer the user's identity. in other words1 let N(t) denote the number of characters typed on a keyboard; the conditional prof of N{t] given A = J. be Poisson: Pmll= (:1) e'\" forn=,1,2,.... Let a spy measure random variable X denoting the time it took for the user to type up the rst character. a. Find E(X|J't = 300) as well as IEUL' I11 = 600) (write your answer in milliseconds)