A random customer buys either brand A, brand B, or brand C with probabilities 0.25, 0.30, 0.35. There is a 0.10 probability that the customer will not buy anything. a) of the next 4 customers, what is the probability that there will be at least one purchase each for brands A, B and C? b) What is the probability that the 3rd purchase of brand B is made before the 6th customer makes his purchase?