4. (4 marks) The number, N, of people who eventually hear a rumour spread by the mass media by time t, in days, is given by the formula N = a1 - e-kt) for positive numbers a and k. Suppose 5,000,000 people hear a rumour eventually. Further, 10% of them heard the rumour in the first day. Find the values of a and k. What percentage of people heard the rumour on the second day?